Expose seats_in_use in namespace entity

What does this MR do?

This MR aligns the EE namespace entity with the changes introduced in !42293 (merged) where the values for seats_in_use and max_seats_used are returned from the gitlab_subscription object (instead of recalculating each time).

!42644 (merged) introduced the max_seats_used attribute. Now seats_in_use is added so it can be used in https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/2082 to display each namespace's seats_in_use on the Admin page.

Related issue

https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/2063

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team