Support Metadata V16

[jsdw]

Subxt currently supports V14 and V15 metadata. V16 metadata is not far off being ready (see paritytech/polkadot-sdk#4520).

To support V16 metadata we'll want to:

• Add support for converting our internal metadata repr to/from V16 metadata (eg like https://github.com/paritytech/subxt/blob/master/metadata/src/from_into/v15.rs for v15).
• Update our artifacts to be V16.
• Update our internal metadata representation to support V16 features, and fix From V14/V15 impls to handle this.
• Ensure our CLI tool can trim pallets etc on V16 metadata (currently only V14 and V15 supported for this: https://github.com/paritytech/subxt/blob/master/cli/src/commands/metadata.rs#L51).
  • Might be worth fixing the error here to say something like "We can only strip pallets or runtime APIs from V14, V15 and V16 metadata" too)

I think with those changes, Subxt will fully support V16 metadata.