Include the `is_admin` field in the `GET /users/:id` API when current user is an admin

What does this MR do?

When current user is an admin, include the is_admin field in the response to GET /users/:id.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #36213 (closed)