Adding an additional ArduinoMqttClient constructor as well as a method setClient to allow late initialisation.

[aentinger]

This allows for the ArduinoIoTCloud firmware to intantiate ArduinoMqttClient on the stack instead of the heap. So instead of

ArduinoMqttClient mqtt_client(another_client_interface);

you can do

ArduinoMqttClient mqtt_client(nullptr);
/* ... some time later the other client interface actually is available */
mqtt_client.setClient(late_available_client_interface);