v16: Expose the unstable metadata v16

[lexnv]

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: #5274

Closes: #5980
Closes: #5347
Closes: #5285

cc @paritytech/subxt-team

[paritytech-workflow-stopper]

All GitHub workflows were cancelled due to failure one of the required jobs.
Failed workflow url: https://github.com/paritytech/polkadot-sdk/actions/runs/11890930796
Failed job name: test-linux-stable-no-try-runtime

[lexnv]

/cmd prdoc --audience node_dev --bump patch

[github-actions]

Command "prdoc --audience node_dev --bump patch" has failed ❌! See logs here

[lexnv]

/cmd prdoc --audience node_dev --bump patch --force

[lexnv]

@Morganamilo could you please take a look over .github/workflows/check-semver.yml changes? 🙏

I would like to merge this and backport it on the latest release, I think an in-depth fix for semver will follow in: #6549