Add a job to check availability of Big Bang artifacts

As of !884 (merged) we are testing two clean installs (OCI and Git) for MRs. This was added to address issues where artifacts were unavailable due to upstream issues publishing artifacts.

While this works as a solution to ensure we get fast feedback if packages are missing artifacts it would be nice to verify the artifacts exist without needing to do do a full deploy.

At a high level this would involve checking that each tag referenced in the Big Bang umbrella values.yaml actually exists in registry1 and the git repository.

The job should run prior to any clean install, likely in the prevar stage.

Relates to #624 (closed)