Add Endpoint to Retrieve Info About Keystore

[joepetrowski]

Problem: Validators often run multiple clients of which only one is their actual validator. For example, they may have several sentries for each validator, and multiple validators may share sentries.

Validators want a way to query their node's keystore to see if it contains certain keys. This will probably take the form of an author RPC endpoint.

Input: The session keys for a given validator. (Can be obtained with nextKeys(stash: AccountId))

Output: true iff the node contains the private keys associated with the public session keys. false otherwise.