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/traits.rs

@@ -106,10 +106,13 @@ pub trait RuntimePublic: Sized {
 
 	/// Verify that the given signature matches the given message using this public key.
 	fn verify<M: AsRef<[u8]>>(&self, msg: &M, signature: &Self::Signature) -> bool;
+
+	/// Returns `Self` as raw vec.
+	fn to_raw_vec(&self) -> Vec<u8>;
 }
 
 /// A runtime interface for an application's public key.
-pub trait RuntimeAppPublic: Sized  {
+pub trait RuntimeAppPublic: Sized {
 	/// An identifier for this application-specific key type.
 	const ID: KeyTypeId;
 
@@ -136,6 +139,9 @@ pub trait RuntimeAppPublic: Sized  {
 
 	/// Verify that the given signature matches the given message using this public key.
 	fn verify<M: AsRef<[u8]>>(&self, msg: &M, signature: &Self::Signature) -> bool;
+
+	/// Returns `Self` as raw vec.
+	fn to_raw_vec(&self) -> Vec<u8>;
 }
 
 /// Something that bound to a fixed `RuntimeAppPublic`.