v16: Expose the unstable metadata v16 (#5732)

This PR exposes the *unstable* metadata V16. The metadata is exposed
under the unstable u32::MAX number.

Developers can start experimenting with the new features of the metadata
v16. *Please note that this metadata is under development and expect
breaking changes until stabilization.*

The `ExtrinsicMetadata` trait receives a breaking change. Its associated
type `VERSION` is rename to `VERSIONS` and now supports a constant
static list of metadata versions.
The versions implemented for `UncheckedExtrinsic` are v4 (legacy
version) and v5 (new version).

For metadata collection, it is assumed that all `TransactionExtensions`
are under version 0.

Builds on top of: https://github.com/paritytech/polkadot-sdk/pull/5274

Closes: https://github.com/paritytech/polkadot-sdk/issues/5980
Closes: https://github.com/paritytech/polkadot-sdk/issues/5347
Closes: https://github.com/paritytech/polkadot-sdk/issues/5285

cc @paritytech/subxt-team

---------

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>
Co-authored-by: Niklas Adolfsson <niklasadolfsson1@gmail.com>
Co-authored-by: Bastian Köcher <git@kchr.de>
Co-authored-by: James Wilson <james@jsdw.me>
Co-authored-by: GitHub Action <action@github.com>