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/offchain/src/lib.rs

@@ -19,8 +19,6 @@
 #![cfg_attr(not(feature = "std"), no_std)]
 #![warn(missing_docs)]
 
-use sp_runtime::traits::NumberFor;
-
 /// Local Storage Prefix used by the Offchain Worker API to
 pub const STORAGE_PREFIX: &[u8] = b"storage";
 
@@ -31,7 +29,7 @@ sp_api::decl_runtime_apis! {
 		/// Starts the off-chain task for given block number.
 		#[skip_initialize_block]
 		#[changed_in(2)]
-		fn offchain_worker(number: NumberFor<Block>);
+		fn offchain_worker(number: sp_runtime::traits::NumberFor<Block>);
 
 		/// Starts the off-chain task for given block header.
 		#[skip_initialize_block]