Add AddOnPurchase to response

What does this MR do and why?

Add AddOnPurchase to response

We extend the mutation userAddOnAssignmentCreate's response to return AddOnPurchase type. This will help client to save extra request to fetch the AddOnPurchase info again, after mutation.

We also remove ability to self-assign from this mutation for now, as it will be worked on different issue/endpoint: https://gitlab.com/gitlab-org/gitlab/-/issues/420138. This makes authorization simple with only admin_add_on_purchase ability check.

Screenshots or screen recordings

How to set up and validate locally

1. Check out the branch
2. Create a new root group namespace
3. Setup some seed records

namespace = Namespace.last
add_on = GitlabSubscriptions::AddOn.find_or_create_by!(name: "code_suggestions") {|e| e.description = "Test"}
add_on_purchase = GitlabSubscriptions::AddOnPurchase.create!(
  add_on: add_on, namespace: namespace, expires_on: 1.month.from_now, quantity: 5, purchase_xid: 'A-S0001'
)

add_on_purchase.to_global_id.to_s # "gid://gitlab/GitlabSubscriptions::AddOnPurchase/9"
namespace.users.first.to_global_id.to_s # "gid://gitlab/User/85"

# enable the feature flag
Feature.enable(:hamilton_seat_management)

4. Go to graphql explorer (logged in as owner/admin): http://gdk.test:3000/-/graphql-explorer, and use following query and variables

mutation {
  userAddOnAssignmentCreate(
    input: {
      userId: "gid://gitlab/User/85",
      addOnPurchaseId: "gid://gitlab/GitlabSubscriptions::AddOnPurchase/9"
    }) {
    addOnPurchase {
      id
      name
      purchasedQuantity
      assignedQuantity
    }
    errors
  }
}

5. The response should now have additional addOnPurchase object attributes
6. If we log out as owner, and log in as developer/guest user, then perform the above query it should return null

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.

Related to #419098