rpc-http-threads cli arg (#8890)

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

* Update `http::ServerBuilder`threads

substrate/client/service/src/config.rs

@@ -89,6 +89,8 @@ pub struct Configuration {
 	pub rpc_ipc: Option<String>,
 	/// Maximum number of connections for WebSockets RPC server. `None` if default.
 	pub rpc_ws_max_connections: Option<usize>,
+	/// Size of the RPC HTTP server thread pool. `None` if default.
+	pub rpc_http_threads: Option<usize>,
 	/// CORS settings for HTTP & WS servers. `None` if all origins are allowed.
 	pub rpc_cors: Option<Vec<String>>,
 	/// RPC methods to expose (by default only a safe subset or all of them).