refactor: introduce authorize_with_unit_primitive_header

What does this merge request do and why?

This MR introduces authorize_with_unit_primitive_header decorator so that we can DRYs up the authorization logic.

We'll iterate on the authorizer for https://gitlab.com/gitlab-org/modelops/applied-ml/code-suggestions/ai-assist/-/issues/487+.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.