GitLab-rails: Support raw artifacts (EE)

What does this MR do?

This MR prepares backend logic for multiple-artifacts

For the test purpose, it's already allowing junit keyword.

For the test purpose, we're testing with EE version which is used on production. This should be prepared at CE level.

---

This is a part of https://gitlab.com/gitlab-org/gitlab-ce/issues/46809

• Gitlab-rails: here
• Gitlab-workhorse: gitlab-workhorse!269 (closed)
• Gitlab-runner: gitlab-runner!923 (merged)

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

Why was this MR needed?

Screenshots (if relevant)

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 Backend maintainer
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Related:

• https://gitlab.com/gitlab-org/gitlab-ce/issues/46809
• gitlab-runner!923 (merged)