Introduces author_hasKey and author_hasSessionKeys rpc endpoints (#4720)

* Introduces `author_hasKey` and `author_hasSessionKeys` rpc endpoints

Both endpoints can be used to check if a key is present in the keystore.

- `hasKey` works on with an individual public key and key type. It
checks if a private key for the given combination exists in the
keystore.
- `hasSessionKeys` works with the full encoded session key blob stored
on-chain in `nextKeys`. This requires that the given blob can be decoded
by the runtime. It will return `true`, iff all public keys of the
session key exist in the storage.

Fixes: https://github.com/paritytech/substrate/issues/4696

* Update client/rpc-api/src/author/error.rs

Co-Authored-By: Nikolay Volf <nikvolf@gmail.com>

* Indentation

Co-authored-by: Nikolay Volf <nikvolf@gmail.com>

substrate/client/rpc-api/src/author/error.rs

@@ -54,6 +54,9 @@ pub enum Error {
 	/// Some random issue with the key store. Shouldn't happen.
 	#[display(fmt="The key store is unavailable")]
 	KeyStoreUnavailable,
+	/// Invalid session keys encoding.
+	#[display(fmt="Session keys are not encoded correctly")]
+	InvalidSessionKeys,
 }
 
 impl std::error::Error for Error {