SOC Module bmwbc improvements

[rleidner]

UI PR: openWB/openwb-ui-settings#656

• instantiate Api and bimmer_connected objects only once and reuse them as long as owb2 runs
  this runs the soc module in a similar way as the bimmer_connected adapter in home assistant
• on debug level log store content when loaded or written (user request)
• optimize import to required modules
• when soc request fails, raise exception to show proper error in vehicle on Status page
• soc_timestamp in CarState to LastUpdatedAt from BMW Server
• implement secondary instance model to improve support of multiple vehicles