instead of iterating a map with subscriptions, we now bind the target
variable to the callback, which is executed once a message is arrived.
this way, the target variable is already linked to the respective topic
when the callback is executed.
lock the mutex when writing the variable, as the MQTT callback is
executed in a different context (MQTT task) than the main loop task,
which otherwise accesses the variables.
it is important to separate the capabilities of each power meter
provider into their own class/source file, as the providers work
fundamentally different and their implementations must not be
intermangled, which made maintenance and improvements a nightmare
in the past.
