Allow to expose a subset of unsafe RPCs (#5233)

* sc-cli: Use type-safe constructors for RPC/Prometheus interfaces

* service: Simplify rpc handler creation

Could probably be further simplifies once [this][commit] lands.

[commit]: https://github.com/paritytech/jsonrpc/commit/20485387ed06a48f1a70bf4d609a7cde6cf0accf

* service: Streamline some HTTP & WS server start logic

* client: Introduce a simple RPC policy mechanism

* rpc/system: Check unsafe RPCs

* rpc/offchain: Check unsafe RPCs

* rpc/author: Check unsafe RPCs

substrate/client/rpc/src/author/tests.rs

@@ -83,6 +83,7 @@ impl TestSetup {
 			pool: self.pool.clone(),
 			subscriptions: Subscriptions::new(Arc::new(self.runtime.executor())),
 			keystore: self.keystore.clone(),
+			deny_unsafe: DenyUnsafe::No,
 		}
 	}
 }