Decouple `IExternalPowerSink` from `IAEPowerStorage`

[yueh]

IExternalPowerSink is mostly used as wrapper to ensure the power units are converted correctly, but there is no need to have it extend IAEPowerStorage as it will only allow inputting.

The internal implementation can then still wrap an IAEPowerStorage. E.g. to charge inscribers or chargers directly with FE.

Also it should be considered to move it into the internal package as this does really offer anything to an API consumer.