Auto assign the new members a duo pro seat

What does this MR do and why?

This MR is the last piece of https://gitlab.com/groups/gitlab-org/-/epics/13637, tackling https://gitlab.com/gitlab-org/gitlab/-/issues/458119 and https://gitlab.com/gitlab-org/gitlab/-/issues/460250

In this MR we:

• Auto-assign the new members a duo pro seat if the group setting is enabled.
• Document the usage of the feature.

Screenshots or screen recordings

Auto-assign when seats are available:

Screen_Recording_2024-05-06_at_12.22.26

Project members:

Screen_Recording_2024-05-06_at_12.34.14

Sub-group members:

Screen_Recording_2024-05-06_at_12.36.02

When no seats are available, the member is successfully added and no duo pro seats are assigned:

Screen_Recording_2024-05-06_at_12.30.38

How to set up and validate locally

1. As a group owner, pick any root namespace, and validate that the field is not present and the above recording is not applicable.
2. Purchase Duo Pro for that namespace, use rails c directly for easier testing.

     namespace = YOUR_NAMESPACE
     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'
     )
   
   # enable the feature flag
   Feature.enable(:auto_assign_gitlab_duo_pro_seats)

3. Validate that the process matches the recordings
4. Validate that no duo pro seats available is not breaking the member addition process