Metadata V16: Have a vec of supported extrinsic versions

[jsdw]

<=V15 metadatas contain a single extrinsic version field (set to 4 currently).

With V5 extrinsics coming, and runtimes that can support either V4 or V5 extrinsics, we should extens this to be a vec of the extrinsic versions that the runtime supports. This allows dApps/client libsto inspect that vec to understand whether they are able to construct a valid extrinsic for it.

[jsdw]

(I think this might already be done in the code, but wanted to track and make sure either way with the issue, \cc @lexnv)

[bkchr]

paritytech/frame-metadata#82 (comment)

[lexnv]

Added this to frame-metadata pr. It should be exposed in the enablement of V16-unstable PR here.

I think its good to have this issue around to not forget about it, next steps here: release frame-metdata crate, merge associated types (#5274) and then the enablement PR