the MQTT power meter can now process the messages published at the
respective topics as JSON and extract a power value using a JSON path
(same as in HTTP+JSON power meter). additionally, selecting a unit for
the power value as well as an option to invert the value's sign was
added as well, similar to the HTTPS+JSON power meter.
instead of reading the main config's powermeter struct(s), the
individual power meters now are instanciated using a copy of their
respective config. this allows to instanciate different power meters
with different configs. as a first step, this simplifies instanciating
power meters for test purposes.
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.
