Resolve "Foreground verification of job artifacts"

What does this MR do?

Implements foreground verification of artifacts in a manner mirroring that of LFS objects and uploads

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

Artifacts now have a file_sha256 attribute so we can do this, whereas previously we couldn't.

We're not backfilling existing data, meaning that runs will tend to be uniformly Missing checksum at present. This will be addressed in https://gitlab.com/gitlab-org/gitlab-ce/issues/43745

We need a matching EE MR, which will also move things there into a common(ish) directory structure: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4863

Why was this MR needed?

This is only marginally useful for the GCP Migration but may be useful to users on smaller instances.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #43949 (closed)