rpc-http-threads cli arg (#8890)

* Add optional `rpc-http-threads` cli arg

* Update `http::ServerBuilder`threads

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

@@ -262,6 +262,7 @@ fn node_config<G: RuntimeGenesis + 'static, E: ChainSpecExtension + Clone + 'sta
 		rpc_ipc: None,
 		rpc_ws: None,
 		rpc_ws_max_connections: None,
+		rpc_http_threads: None,
 		rpc_cors: None,
 		rpc_methods: Default::default(),
 		prometheus_config: None,