[WIP] Migrate from `SignedExtension` to `TransactionExtension` (#2454)

Follow up effort on top of
https://github.com/paritytech/polkadot-sdk/pull/2280

- [x] implement `TransactionExtension` for all `SignedExtension` `impl`
instances
- [x] remove `AsTransactionExtension` adapter
- [x] mark `SignedExtension` as deprecated and remove all
`SignedExtension` impl` instances
- [x] clean up any `SignedExtension` remnants in docs or other names
- [ ] fix broken migration tests?

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Signed-off-by: georgepisaltu <[email protected]>
Co-authored-by: Gavin Wood <[email protected]>