Convert create_user_ns_outside_model FF to actor based

Moving the create_user_ns_outside_model feature flag to an actor based FF bound to the current request.

From the docs:

To migrate an existing feature from percentage of time to the current request actor, it is recommended that you create a new feature flag. This is because it is difficult to control the timing between existing percentage_of_time values, the deployment of the code change, and switching to use percentage_of_actors.