Persist sensor data defined in flexibility models sent with /schedules/trigger [POST]

[Flix6x]

If a soc-sensor entity address is passed, we could persist SoC values, incorporated within the flex model that is sent along with the scheduling trigger, to the corresponding sensor. This would save the need to record SoC values with separate messages.

• [ ] Save current SoC value.
• [ ] Save target SoC values.
• [ ] Attend to the todo in api/v3_0/sensors.py.
• [ ] Also update the note in posting_data.rst about flexibility states not being persisted.