Adjust Duo Pro assign modules to be adaptable to other duo types

What does this MR do and why?

This MR refactors DuoPro bulk assignment modules to be more generic to adapt to other Duo types(Ex: Duo Enterprise). The assignment events are supposed to be identical for duo assignments so they can share the same modules.

Bulk un-assignment modules through gitlab_subscriptions/user_add_on_assignments/bulk_remove.rb will be refactored in a different follow-up MR.

Screenshots or screen recordings

No frontend changes

How to set up and validate locally

• Visit Duo Pro in Admin section of Self-managed instance, or namespace's Duo Pro section of a Saas instance
• Bulk assignment of users should work as intended.
• Make sure the right feature flags are enabled, detailed steps can also be found here
  • For SM, self_managed_code_suggestions and sm_duo_pro_bulk_user_assignment
  • For Saas, hamilton_seat_management and gitlab_com_duo_pro_bulk_user_assignment

Related to #468175