rpc-http-threads cli arg (#8890)

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

* Update `http::ServerBuilder`threads

substrate/test-utils/test-runner/src/utils.rs

@@ -124,6 +124,7 @@ pub fn default_config(task_executor: TaskExecutor, mut chain_spec: Box<dyn Chain
 		rpc_ws: None,
 		rpc_ipc: None,
 		rpc_ws_max_connections: None,
+		rpc_http_threads: None,
 		rpc_cors: None,
 		rpc_methods: Default::default(),
 		prometheus_config: None,