[SPIKE]: Identify seat assignment upsert locations

This issue is part of the effort [Seat Assignment Model] Backend Foundation (&16982)

We figured out that we want to have full coverage of seat assignments for all non-billable and billable members.

We will focus first on GitLab.com and then on Self-Managed/Dedicated.

For this effort, we need to collect all locations where we should create or update seat assignments. For example:

• Users get added to a group.
• Users get added to a project.
• LDAP directory syncs with GitLab.com.
• User logs in after being invited.
• User receives approval.