Support defining soc-max and soc-min as sensors

[Flix6x]

Update the soc-max and soc-min fields in the flex-model to use the QuantityOrSensor schema. For backwards compatibility, it should remain possible to specify a numeric value, in which the case the unit should be taken from the soc-unit field.

Rationale for supporting dynamic values for soc-max and soc-min:

• model battery degradation
• model a future expansion of storage capacity
• update protection thresholds for battery usage (soc-maxima and soc-minima could also be used for this, but those typically deal with highly dynamically changing operational parameters rather than physical thresholds)

Follow-up: as a sensor, soc-max and soc-min could possibly be modelled as instantaneous sensors with event_resolution=timedelta(0), to be treated as setpoints that should remain valid until the next record, rather than as sensors with a non-zero resolution. This would require a change in our search logic, in combination with a dedicated sensor attribute. But for the scope of this issue, I recommend just using a non-zero resolution with a long duration (e.g. a month or a year, with event_resolution="P1Y").