feat(handlers): enable migration mode to be paused

This MR allows the migration to be paused. This is accomplished by checking the new storage prefix for the presence of the repository with the auth eligibility flag is not set before returning the not set status, ensuring that once a repository is present on the new code path, we'll service requests to and from it correctly.

Right now, the auth mock is only available to the API integration tests. We could choose, now or in a future MR, to expand this to the main registry binary for local ad-hoc testing, potentially removing the autheligibilitydisabled parameter in favor of configuring the eligibilitymock auth service.