ESP32 MQTT / UART support code

This code is shared as part of the Embedded Systems Design course at Arizona State University.

Various parts of the code have been derived from the following:

• https://github.com/peterhinch/micropython-async/blob/master/v3/as_demos/auart.py
• https://github.com/tve/mqboard/blob/master/mqtt_async/hello-world.py
• https://github.com/peterhinch/micropython-mqtt
• https://github.com/embedded-systems-design/external_pycopy-lib