rpc-http-threads cli arg (#8890)

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

* Update `http::ServerBuilder`threads

substrate/client/cli/src/commands/run_cmd.rs

@@ -122,6 +122,10 @@ pub struct RunCmd {
 	#[structopt(long = "ws-max-connections", value_name = "COUNT")]
 	pub ws_max_connections: Option<usize>,
 
+	/// Size of the RPC HTTP server thread pool.
+	#[structopt(long = "rpc-http-threads", value_name = "COUNT")]
+	pub rpc_http_threads: Option<usize>,
+
 	/// Specify browser Origins allowed to access the HTTP & WS RPC servers.
 	///
 	/// A comma-separated list of origins (protocol://domain or special `null`
@@ -376,6 +380,10 @@ impl CliConfiguration for RunCmd {
 		Ok(self.ws_max_connections)
 	}
 
+	fn rpc_http_threads(&self) -> Result<Option<usize>> {
+		Ok(self.rpc_http_threads)
+	}
+
 	fn rpc_cors(&self, is_dev: bool) -> Result<Option<Vec<String>>> {
 		Ok(self
 			.rpc_cors