`TransactionExtension`: Update naming

See eg https://github.com/paritytech/polkadot-sdk/pull/2280 and my comment here links to the bit showing the new encoding: https://github.com/paritytech/polkadot-sdk/pull/2280#discussion_r1481193578 (though note that the actual extrinsic version will change to 5 for the new type based on latest comments).

We've used frame-decode to suppor tthis new TX version in Subxt, but we should still:

1. Rename our SignedExtensions to TransactionExtensions (updating the other naming around these as appropriate, eg I think "additional signed" becomes "implicit").
   2 Rename our tx client methods to match the update, eg create_unsigned becomes create_bare.
2. Modify our existing methods to enable creating V5 extrinsics when they are supported (I think this will rely on V16 metadata to show support, and also likely on a TransactionExtension existing which allows us to add a signature.) We want to be able to create "bare" and "general" extrinsics in general, and fall back to creating V4s when V5 not supported.

Start with the linked Polkadot SDK pull request and its encoding discussion, then inspect Subxt's SignedExtension names and tx client methods such as create_unsigned. Trace the existing frame-decode support and V16 metadata handling. Done means the naming is updated, bare and general extrinsics can be created, and V4 remains the fallback when V5 is unavailable.