Add optimistic config flag to modbus select. (#3267)

esphome/components/modbus_controller/select/modbus_select.cpp

@@ -80,6 +80,9 @@ void ModbusSelect::control(const std::string &value) {
   }
 
   parent_->queue_command(write_cmd);
+
+  if (this->optimistic_)
+    this->publish_state(value);
 }
 
 }  // namespace modbus_controller