this changeset adds support for parsing the MQTT battery provider's SoC
and voltage topics' payloads as JSON to extract a numeric value at a
configurable path.
we previously used the mutex to protect writing the target variable.
however, we would only do that for the old usecase, where a plain float
value in Watts was expected as the topic's payload.
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.
all power meter providers now have their own configuration struct
defined. a respective method to serialize and deserialize the provider
config is implemented for each provider.
"powertotal" is always published and it is published by the base class
directly. other values are still published by the derived classes, but
use a base class method, which takes care that a common base topic is
used in particular.
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.
