PageSize selection to Duo add on eligible users list

What does this MR do and why?

Relates to #442149 (closed)

With the goal of making it easy for customers to manage their Duo assignments, we plan to display a dropdown with the ability to choose between 20/50/100 user items. This will also allow them to assign/unassign users based on the selected number of user items. This MR implements this functionality from a frontend perspective, whereas backend is covered by #500036 (closed)

Note: The backend currently only allows up to 50 user assignments/unassignments, the limit extention to 100 is about to go live soon through a different MR. It should be safe to merge this MR as the feature is behind a feature flag and will only be rolled out when all the components are merged.

Screenshots or screen recordings

Bulk selection and assignment	Screen Recording 2024-10-23 at 10.56.11.mov
Bulk unassignment	Screen Recording 2024-10-23 at 11.07.38.mov

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Needs both gdk and CustomersDot running for SaaS/SM (setup guide)
2. In Rails console, enable the FF Feature.enable(:enable_add_on_users_pagesize_selection)

To assign Code Suggestions seats to your group:

For Saas:

1. In Rails console, add_on = GitlabSubscriptions::AddOn.find_or_create_by!(name: "code_suggestions") {|e| e.description = "Test"}
2. In Rails console, GitlabSubscriptions::AddOnPurchase.create!(add_on: add_on, namespace: Namespace.find(<you-name-space-id>), expires_on: 21.month.from_now, quantity: 100, purchase_xid: 'A-S0002')
3. Visit any group or project member pages such as http://localhost:3000/groups/flightjs/-/usage_quotas#code-suggestions-usage-tab
4. The eligible users list should display a new dropdown to choose the number of user items.

For SM:

1. In Rails console, add_on = GitlabSubscriptions::AddOn.find_or_create_by!(name: "code_suggestions") {|e| e.description = "Test"}
2. In Rails console, GitlabSubscriptions::AddOnPurchase.create!(add_on: add_on, expires_on: 21.month.from_now, quantity: 5, purchase_xid: 'A-S0002')
3. Visit admin section's Duo area http://localhost:3000/admin/code_suggestions
4. The eligible users list should display a new dropdown to choose the number of user items.

Related to #442149 (closed)