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/session/src/lib.rs

@@ -25,6 +25,8 @@ use sp_runtime::{generic::BlockId, traits::Block as BlockT};
 #[cfg(feature = "std")]
 use sp_api::ProvideRuntimeApi;
 
+use sp_core::crypto::KeyTypeId;
+
 sp_api::decl_runtime_apis! {
 	/// Session keys runtime api.
 	pub trait SessionKeys {
@@ -36,6 +38,11 @@ sp_api::decl_runtime_apis! {
 		///
 		/// Returns the concatenated SCALE encoded public keys.
 		fn generate_session_keys(seed: Option<Vec<u8>>) -> Vec<u8>;
+
+		/// Decode the given public session keys.
+		///
+		/// Returns the list of public raw public keys + key type.
+		fn decode_session_keys(encoded: Vec<u8>) -> Option<Vec<(Vec<u8>, KeyTypeId)>>;
 	}
 }