Original PR https://github.com/paritytech/polkadot-sdk/pull/2280
reverted in https://github.com/paritytech/polkadot-sdk/pull/3665

This PR reintroduces the reverted functionality with additional changes,
related effort
[here](https://github.com/paritytech/polkadot-sdk/pull/3623).
Description is copied over from the original PR

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 ...