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

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