Use agent user fields as trigger conditions

Feature Request Summary: 

Zendesk Support should allow agent custom user fields as trigger conditions, the same as it currently does for requesters, specifically as the assignee of a ticket.

Description/Use Cases: 

One example use case (of ours) is that we have some agents who want to not receive a specific type of notification, due to volume. It would be great for them to use a checkbox in their profile to turn this on/off, because in the trigger, we could say "Checkbox = checked." This is possible for requesters, but not agents/assignee/etc.

Business impact of limitation or missing feature:

In the example above, we instead have to offer a workaround by instructing users on how to filter out a specially crafted message from Zendesk, instead of just being able to turn the feature on/off at their leisure. The absence of such a feature simply leads to an increase in agent work.