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/bin/node/runtime/src/lib.rs

@@ -798,6 +798,12 @@ impl_runtime_apis! {
 		fn generate_session_keys(seed: Option<Vec<u8>>) -> Vec<u8> {
 			SessionKeys::generate(seed)
 		}
+
+		fn decode_session_keys(
+			encoded: Vec<u8>,
+		) -> Option<Vec<(Vec<u8>, sp_core::crypto::KeyTypeId)>> {
+			SessionKeys::decode_into_raw_public_keys(&encoded)
+		}
 	}
 }