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/primitives/core/src/testing.rs

@@ -127,6 +127,10 @@ impl crate::traits::BareCryptoStore for KeyStore {
 	fn password(&self) -> Option<&str> {
 		None
 	}
+
+	fn has_keys(&self, public_keys: &[(Vec<u8>, KeyTypeId)]) -> bool {
+		public_keys.iter().all(|(k, t)| self.keys.get(&t).and_then(|s| s.get(k)).is_some())
+	}
 }
 
 /// Macro for exporting functions from wasm in with the expected signature for using it with the