Add chunk size constant

lib/MQTT/MqttClient.h

@@ -8,6 +8,10 @@
 #define MQTT_CONNECTION_ATTEMPT_FREQUENCY 5000
 #endif
 
+#ifndef MQTT_PACKET_CHUNK_SIZE
+#define MQTT_PACKET_CHUNK_SIZE 128
+#endif
+
 #ifndef _MQTT_CLIENT_H
 #define _MQTT_CLIENT_H