Resolve "Project exports fail when uploads have been migrated to object storage"

Adds object storage (or general uploader) support to Import/Export project uploads.

• It uses the old logic if the object storage flag is set to false
• Avatar was being restored using an uploader, but not exported (added that)
• Project uploads exports/imports were using local storage only, now it uses uploaders

---

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• Conform by the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #48745 (closed)