Adds seat_type enum to subscription_seat_assignments

What does this MR do and why?

Adds seat_type enum to subscription_seat_assignments.

This will be used to support different seat types for seat assignments.

Currently the known list of seat types is:

• Base - billable seat type
• Free - free tier seat type
• System - bot users (non-billable)
• Plan - Enterprise Agile Planning etc.

References

• https://gitlab.com/gitlab-org/gitlab/-/issues/522974
• https://gitlab.com/gitlab-org/fulfillment/meta/-/issues/1967

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