Add MQTT broker mock to test backend MQTT client

[GodVenn]

Describe the new feature you would like to see
Add a mock of the MQTT broker in the backend to allow for testing of MQTT Event Handler behavior.

Describe the solution you'd like
A mock MQTT broker publishing topics with mock messages so that we can test different behaviors.
E.g. missing fields in topic message or similar.

How will this feature affect the current Threat Model?
Leave blank for maintainers to fill out if you are uncertain about this.

[mrica-equinor]

I need help on this when people are available

[aeshub]

This issue has automatically been marked as stale as there has been no activity for 60 days.

[oysand]

Try to address the need here with integrations testing instead