Stabilize V15 Metadata

[lexnv]

This PR stabilizes the V15 metadata.

Metadata V15 changes

• Metadata V15: Expose API to fetch metadata for version #13287

  • Expose APIs to easily update the metadata in the future. This includes metadata_at_version and metadata_versions
  • Unstable metadata is included under version u32::MAX until stabilization

• Metadata V15: Add Runtime API metadata #13302

  • Collect type information about the runtime API functions.
  • Allows users to generate APIs to call directly into the runtime Metadata V15: Generate Runtime APIs subxt#918

• Metadata V15: Expose types for the overarching Call, Event, Error enums #14143

  • Add overarching Call, Error and Event enums to the metadata to ease decoding.
  • To achieve, an extra step of expansion is added to the construct_runtime! for it to propagate default pallet parts

• Metadata V15: Enrich extrinsic type info for decoding #14123

  • Modify the extrinsic metadata to include type information for Call, Address, Signature and Extra types

• V15: Add custom values to the metadata frame-metadata#61

  • Adds support for custom types without breaking changes.
  • [Meta/PM] Create a deprecation process polkadot-sdk#182 would use these fields to help the deprecation process
  • Could be used for parachains to include specific information

Closes: #12939

@paritytech/subxt-team

[lexnv]

bot rebase

[paritytech-processbot]

Rebased

[lexnv]

bot rebase

[paritytech-processbot]

Branch is already up-to-date