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