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.
this new class uses the newly introduced HttpRequestConfig and performs
HTTP requests using this config. it will be reused for other power
meters (SML over HTTP(S)) and may be reused by other features in the
future (battery provider, solar power provider, etc.).
