Metadata V16: Features to include in V16

[lexnv]

Raising this issue to discuss about the features that are needed by the community in the next metadata version.

These features will be initially exposed under a metadata unstable versioning (u32::max), that will stabilize into v16.

I've compiled a possibly incomplete list of requested features that we may want to add to v16:

### Requested features V16
- [ ] https://github.com/paritytech/polkadot-sdk/issues/4519
- [ ] https://github.com/paritytech/polkadot-sdk/issues/4098
- [ ] https://github.com/paritytech/polkadot-sdk/issues/349
- [ ] https://github.com/paritytech/polkadot-sdk/issues/3594
- [ ] https://github.com/paritytech/polkadot-sdk/issues/3238
- [ ] https://github.com/paritytech/polkadot-sdk/issues/5285
- [ ] https://github.com/paritytech/polkadot-sdk/issues/5347
- [ ] https://github.com/paritytech/polkadot-sdk/issues/5468
- [ ] https://github.com/paritytech/polkadot-sdk/issues/5980

cc @jsdw @niklasad1 @bkchr @paritytech/subxt-team @xlc @josepot

[bkchr]

https://github.com/polkadot-fellows/RFCs/pull/99 with we would need to change the metadata to return multiple sets of transaction/signed extensions indexed by the version.

[josepot]

IMO an absolute must have of metadata v16 is a dedicated top-level entry named "hasher" (right after the "extrinsic" entry for instance, or at least at the same level), which should have 2 different properties:

• "name": the name of the hashing function
• "runtimeApi": a path to a runtime-api which just implements the hasher (E.g: Core_hash)

[Polkadot-Forum]

This issue has been mentioned on Polkadot Forum. There might be relevant details there:

https://forum.polkadot.network/t/metadata-v16-updates/9557/1

[Polkadot-Forum]

This issue has been mentioned on Polkadot Forum. There might be relevant details there:

https://forum.polkadot.network/t/metadata-v16-updates/9557/7