Expose state_prove RPC to generate multiproof

[ishantiw]

Description

We need a way to access stateDB and generate proofs for state's sparse merkle tree. Since we cannot access it outside the application, we need a RPC that can generate it for the requestor.

Expose state_prove RPC under state namespace, since this is the first RPC under this namespace so need to initialize class StateEndpoint similar to ConsensusEndpoint under the same folder.

Acceptance Criteria

• Should have all the unit test for the RPC
• Should be tested with dpos-mainchain to see if its working with api-client

Additional Information

Related discussion: #7438 (comment)