[offchain] Support for sign & verify for crypto keys (#3023)

* Implement sign & verify.

* Use phrases and password.

* Sign & verify with authority keys.

* Fix tests.

* WiP

* WiP

* Allow the caller to decide on 'CryptoKind'.

* Remove TODO.

* Make seed private back.

* Fix non-std build and bump version.

* Use Into<u32> instead of asses.

* Add missing typedef.

substrate/core/service/test/src/lib.rs

@@ -194,7 +194,7 @@ fn node_config<F: ServiceFactory> (
 		offchain_worker: false,
 		force_authoring: false,
 		disable_grandpa: false,
-		password: "".to_string(),
+		password: "".to_string().into(),
 	}
 }