cuprated: internal signatures required for RPC

[hinto-janai]

What

Adds the {function, request, response} signatures needed by cuprated's RPC system to the {blockchain, txpool, p2p, blockchain_context} APIs.

Described in https://github.com/Cuprate/cuprate/issues/279.

This covers the signatures needed for all RPC requests (JSON-RPC, binary, other).