Improve the Users API specs by asserting against the response schema

What does this MR do?

This adds schema checks for the /users and /users/:id API endpoints.

Does this MR meet the acceptance criteria?

• 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