Standardize RPC defintions

[jordy25519]

Move away from the getStorage/state rpc's and make some simply typed RPC interfaces which are easy to consume and support long term.
Things like api.query.genericAsset.freeBalance while they are powerful pushes lots of complexity onto sdks and is really just leaking implementation details like the storage codec, hashes and structure.

It should be easy to curl a balance, without requiring the SCALE codec at all for encoding or decoding, the client should handle this.

Ideas:

• [x] Investigate generative approaches e.g. grpc + JSON
• [ ] Document and implement 0-SCALE interfaces for common queries e.g. (get_block, get_balance, get_nft_attributes)