Is your feature request related to a problem? Please describe.
Our realtime data is only available in the SiriET format via MQTT. We don't want to have to transform it to the supported update methods for SiriET. GTFS-RT unfortunately does not support everything we need.

Goal / high level use-case
Update realtime via SiriET and MQTT

Describe the solution you'd like
We want to write a SiriETMqttUpdater implementing the GraphUpdater, similarly to the existing SiriETGooglePubsubUpdater.

Describe alternatives you've considered
Using a GooglePubSub System. This would lead to additional infrastructure needed, and more elapsed time for every realtime delivery.

Additional context
GTFS-RT has an MQTT updater, the implementation would be similar.