Add dsmr_reader telegram timestamp and device classes (#42909)
* Added Telegram timestamp
* added to 'dsmr/reading/timestamp' and 'dsmr/consumption/gas/read_at'
* Fixed import sorting
* Replaced 'kW' with ENERGY_KILO_WATT_HOUR
* Added device_class, changed unit_of_measurement with fallback on device_class
* Fixed typo
* Fixed 'black' coding format
* Removed fallback, added unit_of_measurement and CURRENCY_EURO as device_class
* Fixed newline
* Removed 'timestamp' unit_of_meassure
* Removed icons from defintions with device_class
* Updated device_classes
* Updated device_classes
* Updated device_classes
* Added 'entity_registry_enabled_default' properties
* Added 'entity_registry_enabled_default' properties, fixed typo
* MQTT discovery will be in another pull-request
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