the parameters to peform an HTTP request by the HTTP(S)+JSON power meter
have been generalized by introducing a new config struct. this is now
used for all values which the HTTP(S)+JSON power meter can retrieve, and
also used by the HTTP+SML power meter implementation. we anticipate that
other feature will use this config as well.
generalizing also allows to share serialization and deserialization
methods in the configuration handler and the web API handler, leading to
de-duplication of code and reduced flash memory usage.
a new web UI component is implemented to manage a set of HTTP request
settings.
"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.
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.
