Remove hard-coded battery capacity from modem logic

[ArendJanKramer]

Currently the modem code is hard-coded to 28kWh.

Todo:

• Adjust callback endpoint of SmartEVSE, to accept battery capacity as input
• Adjust pyPLC, so extract battery information from modem

Please assign to myself

[arpiecodes]

https://github.com/serkri/SmartEVSE-3/pull/160 should solve this issue ✅

Of course feedback always welcome! Especially on my creative MODEM_DENIED state.