Move MDNS to DNS-SD lib, fix init, add example app

Also add a hwtest for ESP8266 and ESP32

CL: Move MDNS to DNS-SD lib, fix init, add example app

PUBLISHED_FROM=9184c86afc03ccf4ca3043c1e60d024a2816a68e

Author: Deomid Ryabkov

src/mgos_mqtt.c

@@ -14,7 +14,6 @@
 #include "common/queue.h"
 #include "mgos_debug.h"
 #include "mgos_event.h"
-#include "mgos_mdns.h"
 #include "mgos_mongoose.h"
 #include "mgos_net.h"
 #include "mgos_sys_config.h"