Use custom User-Agent header in GCP Client requests

What does this MR do?

Adds a custom User Agent header to all GCP Client API requests.

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

N/A

Should there be a changelog entry for this?

Why was this MR needed?

Screenshots (if relevant)

N/A

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• N/A Documentation created/updated
• N/A API support added
• Tests added for this feature/bug
• Review
  • N/A Has been reviewed by UX
  • N/A Has been reviewed by Frontend
  • Has been reviewed by Backend
  • N/A Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• N/A Internationalization required/considered

What are the relevant issue numbers?

Closes #40583