Closes #2160

First part of [Extrinsic
Horizon](https://github.com/paritytech/polkadot-sdk/issues/2415

)

Introduces a new trait `TransactionExtension` to replace
`SignedExtension`. Introduce the idea of transactions which obey the
runtime's extensions and have according Extension data (né Extra data)
yet do not have hard-coded signatures.

Deprecate the terminology of "Unsigned" when used for
transactions/extrinsics owing to there now being "proper" unsigned
transactions which obey the extension framework and "old-style" unsigned
which do not. Instead we have __*General*__ for the former and
__*Bare*__ for the latter. (Ultimately, the latter will be phased out as
a type of transaction, and Bare will only be used for Inherents.)

Types of extrinsic are now therefore:
- Bare (no hardcoded signature, no Extra data; used to be known as
"Unsigned")
- Bare transactions (deprecated): Gossiped, validated with
`ValidateUnsigned` (deprecated) and the `_bare_compat` bits of
`TransactionExtension` (deprecated).
  - Inherents: Not gossiped, validated with `ProvideInherent`.
- Extended (Extra data): Gossiped, validated via `TransactionExtension`.
  - Signed transactions (with a hardcoded signature).
  - General transactions (without a hardcoded signature).

`TransactionExtension` differs from `SignedExtension` because:
- A signature on the underlying transaction may validly not be present.
- It may alter the origin during validation.
- `pre_dispatch` is renamed to `prepare` and need not contain the checks
present in `validate`.
- `validate` and `prepare` is passed an `Origin` rather than a
`AccountId`.
- `validate` may pass arbitrary information into `prepare` via a new
user-specifiable type `Val`.
- `AdditionalSigned`/`additional_signed` is renamed to
`Implicit`/`implicit`. It is encoded *for the entire transaction* and
passed in to each extension as a new argument to `validate`. This
facilitates the ability of extensions to acts as underlying crypto.

There is a new `DispatchTransaction` trait which contains only default
function impls and is impl'ed for any `TransactionExtension` impler. It
provides several utility functions which reduce some of the tedium from
using `TransactionExtension` (indeed, none of its regular functions
should now need to be called directly).

Three transaction version discriminator ("versions") are now
permissible:
- 0b000000100: Bare (used to be called "Unsigned"): contains Signature
or Extra (extension data). After bare transactions are no longer
supported, this will strictly identify an Inherents only.
- 0b100000100: Old-school "Signed" Transaction: contains Signature and
Extra (extension data).
- 0b010000100: New-school "General" Transaction: contains Extra
(extension data), but no Signature.

For the New-school General Transaction, it becomes trivial for authors
to publish extensions to the mechanism for authorizing an Origin, e.g.
through new kinds of key-signing schemes, ZK proofs, pallet state,
mutations over pre-authenticated origins or any combination of the
above.

## Code Migration

### NOW: Getting it to build

Wrap your `SignedExtension`s in `AsTransactionExtension`. This should be
accompanied by renaming your aggregate type in line with the new
terminology. E.g. Before:

```rust
/// The SignedExtension to the basic transaction logic.
pub type SignedExtra = (
	/* snip */
	MySpecialSignedExtension,
);
/// Unchecked extrinsic type as expected by this runtime.
pub type UncheckedExtrinsic =
	generic::UncheckedExtrinsic<Address, RuntimeCall, Signature, SignedExtra>;
```

After:

```rust
/// The extension to the basic transaction logic.
pub type TxExtension = (
	/* snip */
	AsTransactionExtension<MySpecialSignedExtension>,
);
/// Unchecked extrinsic type as expected by this runtime.
pub type UncheckedExtrinsic =
	generic::UncheckedExtrinsic<Address, RuntimeCall, Signature, TxExtension>;
```

You'll also need to alter any transaction building logic to add a
`.into()` to make the conversion happen. E.g. Before:

```rust
fn construct_extrinsic(
		/* snip */
) -> UncheckedExtrinsic {
	let extra: SignedExtra = (
		/* snip */
		MySpecialSignedExtension::new(/* snip */),
	);
	let payload = SignedPayload::new(call.clone(), extra.clone()).unwrap();
	let signature = payload.using_encoded(|e| sender.sign(e));
	UncheckedExtrinsic::new_signed(
		/* snip */
		Signature::Sr25519(signature),
		extra,
	)
}
```

After:

```rust
fn construct_extrinsic(
		/* snip */
) -> UncheckedExtrinsic {
	let tx_ext: TxExtension = (
		/* snip */
		MySpecialSignedExtension::new(/* snip */).into(),
	);
	let payload = SignedPayload::new(call.clone(), tx_ext.clone()).unwrap();
	let signature = payload.using_encoded(|e| sender.sign(e));
	UncheckedExtrinsic::new_signed(
		/* snip */
		Signature::Sr25519(signature),
		tx_ext,
	)
}
```

### SOON: Migrating to `TransactionExtension`

Most `SignedExtension`s can be trivially converted to become a
`TransactionExtension`. There are a few things to know.

- Instead of a single trait like `SignedExtension`, you should now
implement two traits individually: `TransactionExtensionBase` and
`TransactionExtension`.
- Weights are now a thing and must be provided via the new function `fn
weight`.

#### `TransactionExtensionBase`

This trait takes care of anything which is not dependent on types
specific to your runtime, most notably `Call`.

- `AdditionalSigned`/`additional_signed` is renamed to
`Implicit`/`implicit`.
- Weight must be returned by implementing the `weight` function. If your
extension is associated with a pallet, you'll probably want to do this
via the pallet's existing benchmarking infrastructure.

#### `TransactionExtension`

Generally:
- `pre_dispatch` is now `prepare` and you *should not reexecute the
`validate` functionality in there*!
- You don't get an account ID any more; you get an origin instead. If
you need to presume an account ID, then you can use the trait function
`AsSystemOriginSigner::as_system_origin_signer`.
- You get an additional ticket, similar to `Pre`, called `Val`. This
defines data which is passed from `validate` into `prepare`. This is
important since you should not be duplicating logic from `validate` to
`prepare`, you need a way of passing your working from the former into
the latter. This is it.
- This trait takes two type parameters: `Call` and `Context`. `Call` is
the runtime call type which used to be an associated type; you can just
move it to become a type parameter for your trait impl. `Context` is not
currently used and you can safely implement over it as an unbounded
type.
- There's no `AccountId` associated type any more. Just remove it.

Regarding `validate`:
- You get three new parameters in `validate`; all can be ignored when
migrating from `SignedExtension`.
- `validate` returns a tuple on success; the second item in the tuple is
the new ticket type `Self::Val` which gets passed in to `prepare`. If
you use any information extracted during `validate` (off-chain and
on-chain, non-mutating) in `prepare` (on-chain, mutating) then you can
pass it through with this. For the tuple's last item, just return the
`origin` argument.

Regarding `prepare`:
- This is renamed from `pre_dispatch`, but there is one change:
- FUNCTIONALITY TO VALIDATE THE TRANSACTION NEED NOT BE DUPLICATED FROM
`validate`!!
- (This is different to `SignedExtension` which was required to run the
same checks in `pre_dispatch` as in `validate`.)

Regarding `post_dispatch`:
- Since there are no unsigned transactions handled by
`TransactionExtension`, `Pre` is always defined, so the first parameter
is `Self::Pre` rather than `Option<Self::Pre>`.

If you make use of `SignedExtension::validate_unsigned` or
`SignedExtension::pre_dispatch_unsigned`, then:
- Just use the regular versions of these functions instead.
- Have your logic execute in the case that the `origin` is `None`.
- Ensure your transaction creation logic creates a General Transaction
rather than a Bare Transaction; this means having to include all
`TransactionExtension`s' data.
- `ValidateUnsigned` can still be used (for now) if you need to be able
to construct transactions which contain none of the extension data,
however these will be phased out in stage 2 of the Transactions Horizon,
so you should consider moving to an extension-centric design.

## TODO

- [x] Introduce `CheckSignature` impl of `TransactionExtension` to
ensure it's possible to have crypto be done wholly in a
`TransactionExtension`.
- [x] Deprecate `SignedExtension` and move all uses in codebase to
`TransactionExtension`.
  - [x] `ChargeTransactionPayment`
  - [x] `DummyExtension`
  - [x] `ChargeAssetTxPayment` (asset-tx-payment)
  - [x] `ChargeAssetTxPayment` (asset-conversion-tx-payment)
  - [x] `CheckWeight`
  - [x] `CheckTxVersion`
  - [x] `CheckSpecVersion`
  - [x] `CheckNonce`
  - [x] `CheckNonZeroSender`
  - [x] `CheckMortality`
  - [x] `CheckGenesis`
  - [x] `CheckOnlySudoAccount`
  - [x] `WatchDummy`
  - [x] `PrevalidateAttests`
  - [x] `GenericSignedExtension`
  - [x] `SignedExtension` (chain-polkadot-bulletin)
  - [x] `RefundSignedExtensionAdapter`
- [x] Implement `fn weight` across the board.
- [ ] Go through all pre-existing extensions which assume an account
signer and explicitly handle the possibility of another kind of origin.
- [x] `CheckNonce` should probably succeed in the case of a non-account
origin.
- [x] `CheckNonZeroSender` should succeed in the case of a non-account
origin.
- [x] `ChargeTransactionPayment` and family should fail in the case of a
non-account origin.
  - [ ] 
- [x] Fix any broken tests.

---------

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