Add transfer project endpoint to the Projects API

What does this MR do?

This adds a missing API endpoint to transfer a project to another namespace

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

• Should the user_project be reloaded after after the call to Project::TransferService? It happens in Admin::ProjectsController#transfer.

Why was this MR needed?

Requested in #45146 (closed)

Does this MR meet the acceptance criteria?

• 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
• 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 #45146 (closed)