move MQTT_MAX_PACKET_SIZE to pubsubclient library

lib/pubsubclient/src/PubSubClient.h

@@ -23,7 +23,11 @@
 
 // MQTT_MAX_PACKET_SIZE : Maximum packet size
 #ifndef MQTT_MAX_PACKET_SIZE
-#define MQTT_MAX_PACKET_SIZE 512
+	#if defined(ESP8266) || defined(ESP32) || defined(__AVR_ATmega2560__) || defined(__AVR_ATmega1280__)
+	  #define MQTT_MAX_PACKET_SIZE 1024
+	#else // boards with smaller memory
+	  #define MQTT_MAX_PACKET_SIZE 256
+	#endif
 #endif
 
 // MQTT_KEEPALIVE : keepAlive interval in Seconds