feat: implement RPC Discover dynamically

[meowsbits]

Currently the rpc.discover API method returns a static string, fitting the spec documented here: https://github.com/etclabscore/ethereum-json-rpc-specification/blob/master/openrpc.json.

This spec represents a minimum sufficient interface that clients should support.

However this spec does not limit the client from supporting further methods or documenting them via this method.

Instead of multigeth returning a static minimum-viable string at this endpoint, it would be really great to be able to have multigeth actually return a complete representation of the methods that it is making available, which are a lot.

"Handrolling" a specification object to this purpose costs technical debt in the short and long terms, is prone to error, and generally an inelegant way of solving the challenge.

Better, to devise a system that can be applied at the RPC server level, which can programmatically generate an OpenRPC schema fitting the actual API available. This would work, then, hand-in-hand with github.com/gregdhill/go-openrpc (that codebase can consume schemas but not generate them).