Change add-on availability to use UserAssignment

What does this MR do and why?

Addresses: https://gitlab.com/gitlab-org/gitlab/-/issues/419667+

Change add-on availability to use UserAssignment

We want users have access to the code suggestions add on only when they've been assigned a seat by an admin. When the seat-management FF is enabled, check that this user has been assigned a seat to determine their eligibility.

How to set up and validate locally

1. Enable the seat management FF (Feature.enable(:hamilton_seat_management))
2. Create a new group
3. Create an add-on purchase for the group

   group = Group.last
   purchase = GitlabSubscriptions::AddOnPurchase.create!(add_on: GitlabSubscriptions::AddOn.find_or_create_by_name(:code_suggestions), namespace: group, quantity: 10, expires_on: 1.year.from_now, purchase_xid: 'A-12345')

4. Check if your user has access to the add-on

   group.owners.first.code_suggestions_add_on_available?
   => false

5. Assign the add-on to your user

   group.owners.first.assigned_add_ons.create!(add_on_purchase: purchase)

6. Check if your user has access to the add-on

   group.owners.first.code_suggestions_add_on_available?
   => true

SQL Queries

Two queries are made when code_suggestions_add_on_available? is called:

Find the Code Suggestions add-on

Production query plan: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/22955/commands/74002

Local query plan with data: https://explain.depesz.com/s/akdw

SELECT
    "subscription_add_ons"."id"
FROM
    "subscription_add_ons"
WHERE
    "subscription_add_ons"."name" = 1
LIMIT 1;

Find an assignment to the add-on for a user

Production query plan (for my GitLab user without an assignment): https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/22955/commands/74003

Local query plan for a user with lots of assignment data: https://explain.depesz.com/s/gUgQ

SELECT
    1 AS one
FROM
    "subscription_user_add_on_assignments"
    INNER JOIN "subscription_add_on_purchases" ON "subscription_add_on_purchases"."id" = "subscription_user_add_on_assignments"."add_on_purchase_id"
WHERE
    "subscription_user_add_on_assignments"."user_id" = 4981524
    AND (expires_on >= '2023-10-10')
    AND "subscription_add_on_purchases"."subscription_add_on_id" = 1
LIMIT 1;

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.