Add "on user changed" event

In reference to community topic to be found here: Actively trigger actions upon user login/change

Adding an event trigger “on user changed” would allow to trigger certain actions if a user changed at a station mid process.

In the particular use case described in the linked community article, we want to implement additional safety checks once a user changes - but without risking derailing the entire process, as the user change is only allowed to occur at very specific moments in the process.