Add statement-store related JSON-RPC interface to the specification

[AndreiEres]

As we claim that our interface is stable we need to update the specification:
https://github.com/paritytech/json-rpc-interface-spec

[AndreiEres]

@alexggh can you look at that please?

[lexnv]

@alexggh can you look at that please?

Do we have a PR? I could help with reviews 🙏