MQTT API design

[krzmaz]

MQTT support seems to be a feature wanted by at least two people - @anson-vandoren and @fixitfoxy 😁 (See #13)

I'm starting this thread to gather ideas and requirements for a useful implementation of MQTT support (as I'm not using MQTT so I won't be testing the implementation much).
One important thing to consider is the fact that the radio in ESP32 is shared between BT and WiFi. For this reason, we should avoid communication via WiFi (which includes MQTT) while we are sending commands to the desk.
For the most basic support, as described in #6 the goal would be to implement getting and setting height. To achieve that, I see following requirements:

• add four (?) fields to the config page: broker IP, broker port, username, password
• listen on a hardcoded (?) topic for height destinations
• publish the height to a hardcoded (?) topic
  • in idle state we would publish the height periodically - every 10s (?)
  • for moving (both using the button on the desk and using our controller) we would store the last time the height callback was received via BT. After a set amount of time (1s?) without callbacks (movement) we would publish the last received height.

The places marked with question marks are my initial guesses and I'd love some feedback on them.