Partially update rpc endpoint to version 0.15.0

[cptartur]

Usage related changes

Updated RPC endpoints to partially match 0.15 specs:

• added contract class related methods
• added support for declare transactions
• updated return types for existing methods

Not added:

• Some methods from the spec
• "pending", "latest" block support in most methods
• Nonce related parameters always return empty results (no support in cairo-lang/devnet afaik as of now)

Development related changes

None

Checklist:

• Performed a self-review of the code
• Rebased to the base branch
• Documented the changes
• Updated the tests
• All tests are passing

[FabijanC]

• README.md needs to be updated (API version).
• I don't understand the observation about nonces.
• In the meantime, specifications have been updated to v0.16.0 😃 .

[cptartur]

• Readme has been updated.
• What I meant is that the RPC added some kind of native support for "nonces", but I don't think it's possible to support it right now. It has a new endpoint getNonce endpoint. Also nonces field has been added to block state update, and this will always return an empty list for now, because I don't think there's a way to support it yet.
• I've seen 0.16 is out but i think it'll be better to have it as a separate PR.