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/application-crypto/src/lib.rs

@@ -301,6 +301,10 @@ macro_rules! app_crypto_public_common {
 			fn verify<M: AsRef<[u8]>>(&self, msg: &M, signature: &Self::Signature) -> bool {
 				<$public as $crate::RuntimePublic>::verify(self.as_ref(), msg, &signature.as_ref())
 			}
+
+			fn to_raw_vec(&self) -> $crate::Vec<u8> {
+				<$public as $crate::RuntimePublic>::to_raw_vec(&self.0)
+			}
 		}
 	}
 }