Add ability to assign and unassign code suggestions add-on

What does this MR do and why?

Adds ability to assign and un-assign code suggestions add-on on usage quotas page.

This MR is the first of the work needed for #420010.

Description	MR
Add ability to assign and un-assign code suggestions add-on	👈 You are here
Handle errors when assigning and un-assigning	!128320 (merged)
Update code suggestions statistics card with latest counts when assigning and un-assigning add-ons	🚧

All changes are behind enable_hamilton_in_usage_quotas_ui feature flag.

Note: When assigning and un-assigning add-ons, the state is not persisted on refresh. This is because the current add-on assignment status comes from billable_members API and the changes needed to send the add-on status in the billable members list is in progress, so the add-on status is always in OFF state on page load for now.

Related to #420010

Screenshots or screen recordings

Scenario	Screenshot
Assigning and un-assigning code suggestions add-on	Screen recording Screen_Recording_2023-08-02_at_2.34.15_PM
Loading state while assigning	Screenshot
Loading state while un-assigning	Screenshot

How to set up and validate locally

1. Customers Dot needs to be setup in order to view the billing page for a group

2. Start GDK in SaaS mode GITLAB_SIMULATE_SAAS=1 gdk start

3. Create a Group

4. Enable enable_hamilton_in_usage_quotas_ui FF (globally, or for your group)

5. Buy a Subscription for your group from Settings > Billing

6. Visit /groups/<your-namespace-name>/-/usage_quotas#seats-quota-tab

7. Purchase add-on for namespace

   # Get the desired namespace
   namespace = Namespace.last
   # Create an add-on
   add_on = GitlabSubscriptions::AddOn.find_or_create_by!(name: "code_suggestions") {|e| e.description = "Test"}
   # Purchase add-ons for the namespace
   add_on_purchase = GitlabSubscriptions::AddOnPurchase.create!(
     add_on: add_on, namespace: namespace, expires_on: 1.month.from_now, quantity: 2, purchase_xid: 'A-S0001'
   )

8. Add this snippet here to enable add-on assignment. This enables the add-on assignment for the first 2 users in the list (as per assignable value).

    state.members[0].add_ons = { assigned: [], assignable: [{ name: 'CODE_SUGGESTIONS' }] };
    state.members[1].add_ons = { assigned: [], assignable: [{ name: 'CODE_SUGGESTIONS' }] };
    state.members[2].add_ons = { assigned: [], assignable: [] };

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.