Resolve "API endpoint for exporting project"

What does this MR do?

Adds project export endpoints to the API.

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

• Tests: Is it safe to use FileUtils.remove_dir on the export storage path after :suite? Is this cleanup already handled in another way?
• Tests: Are the manual file operations the right way to simulate started and finished exports?

Why was this MR needed?

• Issue #29130 (closed)

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

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

What are the relevant issue numbers?

Closes #29130 (closed)