`pi_heating_demand` feature support

[MiloshCZ]

There is missing property "pi_heating_demand" in zigbee2mqtt domoticz plugin. Log from domoticz:

2021-02-19 06:52:52.230 Error: Python Plugin System: (Zigbee2MQTT) 0x842e14fffe5aa40e: can not process numeric item "pi_heating_demand"
2021-02-19 06:53:33.989 Error: Python Plugin System: (Zigbee2MQTT) 0x842e14fffe5aa40e: can not process numeric item "pi_heating_demand"

Device description Danfoss Ally thermostatic valve

Zigbee2mqtt page https://www.zigbee2mqtt.io/devices/014G2461.html

MQTT message with device information

[{"dateCode":"20201010","friendly_name":"Coordinator","ieeeAddr":"0x00124b00061078e5","lastSeen":1613729520011,"networkAddress":0,"softwareBuildID":"zStack30x","type":"Coordinator"},{"dateCode":"20201215","description":"Ally thermostat","friendly_name":"0x842e14fffe5aa40e","hardwareVersion":69,"ieeeAddr":"0x842e14fffe5aa40e","lastSeen":1613729508890,"manufacturerID":4678,"manufacturerName":"Danfoss","model":"014G2461","modelID":"eTRV0100","networkAddress":56363,"powerSource":"Battery","softwareBuildID":"01.08.0008 01.08","type":"EndDevice","vendor":"Danfoss"}]

MQTT messages from device

{"battery":96,"heat_required":true,"last_seen":1613729508890,"linkquality":55,"local_temperature":24.39,"mounted_mode":false,"occupied_heating_setpoint":24,"pi_heating_demand":6,"setpoint_change_source":2,"update":{"state":"idle"},"update_available":false,"window_open_internal":1}

MQTT state change commands If device supports state change put MQTT topics + payload examples to change device state here