this extends the MqttBattery implementation by an additional topic which
allows to subscribe to receive battery voltage readings through the MQTT
broker. similar to the battery SoC topic, this allows to import a
critical battery data point for the DPL, in case the user chooses to use
voltage thresholds rather than SoC thresholds to control the DPL. if an
otherwise incompatible BMS is available which publishes the battery pack
voltage through MQTT, this can now be used to feed accurate voltage
readings to the DPL.
this battery provider implementation subscribes to a user-configurable
MQTT topic to retrieve the battery SoC value. the value is not
re-published under a different topic. there is no card created in the
web app's live view, since the SoC is already part of the totals at the
top of the live view. that is the only info this battery provider
implements.
closes#293.
relates to #581.
