dmm/bm52x: add support for recorded measurements (memory data sets)

Extend the BM52x packet parser, add config get/set/list code to handle
the data source property. Either let the common serial-dmm code run the
acquisition of live readings, or locally retrieve the selected "session
page" and forward its measurements to the session bus. These separate
code paths are required because the protocol differs a lot between these
modes, a totally different set of requests and responses is involved,
response interpretation logic is impossible to share between live and
recorded measurements.