Map MetricType.TIMESTAMP to SensorDeviceClass.TIMESTAMP

This enables Home Assistant to automatically localize epoch timestamps to the
user's configured timezone instead of displaying them as UTC strings.

Fixes #430

Author: tomer-w

custom_components/victron_mqtt/sensor.py

@@ -48,6 +48,7 @@
     MetricType.DURATION: SensorDeviceClass.DURATION,
     MetricType.ENUM: SensorDeviceClass.ENUM,
     MetricType.IRRADIANCE: SensorDeviceClass.IRRADIANCE,
+    MetricType.TIMESTAMP: SensorDeviceClass.TIMESTAMP,
 }
 
 METRIC_NATURE_TO_STATE_CLASS: dict[MetricNature, SensorStateClass] = {