MQTT: Refactor JSON to find first key
Problem:
A user wants to use Tasmota switches and show Energy levels.
This is currently not supported as the temperature/humidity keys are the only ones supported and also only in a fixed JSON structure.
See https://github.com/sieren/Homepoint/blob/master/main/mqtt/MQTTSensorUtils.hpp L50.

Idea:
This can be refactored to traverse the DOM and find the first occurrence of a given key and use its value. While this will ignore othe, later keys with the same name in the same DOM, this may be a sufficient improvement to usability vs cost of making this work.
(https://stackoverflow.com/questions/55629185/find-all-keys-json-rapidjson)