Implement state RPC methods & error codes

[ansermino]

If this needs to be split into smaller PRs or some of the RPC calls can't be implemented now, please create another task with a list of whatever remains

References:

• https://github.com/w3f/PSPs/blob/master/PSPs/drafts/psp-6.md#111-state
• https://polkadot.js.org/api/substrate/rpc.html

State RPC calls

• State RPC Error Codes
• state_call
• getMetadata
• queryStorage
• subscribeRuntimeVersion
• subscribeStorage
• getReadProof
• getRuntimeVersion

[edwardmack]

• All methods need to implement handling block hash parameter (See issue Implement handling of encoded block hash for storage RPC functions #834)
• All Child calls, need understanding of Child Info and Child type, currently labeled "future-reserved", how should we deal with that?

[danforbes]

@EclesioMeloJunior - can we close this since we have individual issues now?

[EclesioMeloJunior]

@danforbes yes!