Draft: Update billing for group links assigned to a custom role

What does this MR do and why?

• Follows from !159213 (merged)
• In the last MR, we allowed invited groups to be assigned a custom role
• In this MR, we update the member_count when querying a custom role to include members from the invited group
• We also update the billing methods to include invited groups that have been assigned a member role which occupies_seat.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

1. Make you are on an Ultimate license and you have SaaS mode off.
2. In the rails console:

# enable feature
Feature.enable(:assign_custom_roles_to_group_links)

# create a member role
member_role = MemberRole.create(base_access_level: 10, name: "Test", admin_cicd_variables: true)

3. Navigate to any group that you are an owner of. Go to Manage > Members.
4. Click on the Invite a group button and invite any group with the custom role you created in step 2.
5. To confirm that

group_id = GroupGroupLink.last.shared_group_id

Related to #443369