add mqtt connected callback

src/user_code.hpp

@@ -24,6 +24,7 @@ void pwm_changed_user_callback(int pin);
 void ac_changed_user_callback(int mode, int temperature, int fan_speed);
 void timer_tick_callback();
 void timer1_callback();
+void mqtt_connected_user_callback();
 void bt0PopCallback(void *ptr);