Add seat assignment sync service

What does this MR do and why?

Adds a seat assignment sync service to create, update or remove seat assignment records for the top-level namespace.

For each user, the service will:

• Create a seat assignment record for a user with group or project memberships within the namespace hierarchy
• Update an existing seat assignment record for a user with group or project memberships within the namespace hierarchy
• Delete an existing seat assignment record for a user with no group or project memberships within the namespace hierarchy
• Do nothing for a user that do not have an existing seat assignment record and no group or project memberships within the namespace hierarchy

References

• [SAM] Add seat assignment sync service (#570349 - closed)

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