What does this MR do and why?

Resolves: https://gitlab.com/gitlab-org/gitlab/-/issues/443568

We begin to implement bulk users assignment ability for Duo Pro in Self managed instances. Epic: https://gitlab.com/groups/gitlab-org/-/epics/12959. This is very similar to the work that has already been implemented for Gitlab.com through this epic: &11463 (closed)

As a starting step for frontend, this MR creates a new feature flag sm_duo_pro_bulk_user_assignment to allow/disallow admin to bulk assign users from the Admin page on SM instance. An adjustment has also been made to the helper method duo_pro_bulk_user_assignment_available which checks for feature flag option based on the instance being Saas or self managed.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

No frontend changes.

How to set up and validate locally

No functionality has been changed. Nothing to manually test, and the feature flags exposure itself is covered with automated tests

Related to #443568