Add registry migration Guard dynamic pre import timeout

When detecting a long running migration in the pre import step, the
Guard worker will now take into account:
* The pre import tags rate (from the Container Registry dashboards).
  * This is a new application setting.
* The number of tags of the given image repository.

This should lower the amount of false positives we have for stuck long
running migrations.

This change is gated behind a feature flag.

Changelog: added