Implement the author RPC api.

[nistanimirov]

Create the following:

• [x] author_rotateKeys - Used to generate and insert session keys in the host key-store.
• [x] author_insertKey - Used to insert a key into the key-store when it has been manually generated.
• [x] author_hasKey - Checks the key-store for an existing key entry.
• [x] author_hasSessionKeys - Same as hasKey, but for all session keys concatenated.
• [x] author_submitExtrinsic - Submit an extrinsic (e.g., key registration) for the validator to be registered.
• [x] author_submitAndWatchExtrinsic - Same as submitExtrinsic, but a pub-sub