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 {

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

@@ -39,7 +39,8 @@ pub trait AuthorApi<Hash, BlockHash> {
 
 	/// Insert a key into the keystore.
 	#[rpc(name = "author_insertKey")]
-	fn insert_key(&self,
+	fn insert_key(
+		&self,
 		key_type: String,
 		suri: String,
 		public: Bytes,
@@ -49,6 +50,20 @@ pub trait AuthorApi<Hash, BlockHash> {
 	#[rpc(name = "author_rotateKeys")]
 	fn rotate_keys(&self) -> Result<Bytes>;
 
+	/// Checks if the keystore has private keys for the given session public keys.
+	///
+	/// `session_keys` is the SCALE encoded session keys object from the runtime.
+	///
+	/// Returns `true` iff all private keys could be found.
+	#[rpc(name = "author_hasSessionKeys")]
+	fn has_session_keys(&self, session_keys: Bytes) -> Result<bool>;
+
+	/// Checks if the keystore has private keys for the given public key and key type.
+	///
+	/// Returns `true` if a private key could be found.
+	#[rpc(name = "author_hasKey")]
+	fn has_key(&self, public_key: Bytes, key_type: String) -> Result<bool>;
+
 	/// Returns all pending extrinsics, potentially grouped by sender.
 	#[rpc(name = "author_pendingExtrinsics")]
 	fn pending_extrinsics(&self) -> Result<Vec<Bytes>>;