Resolve "Use dynamic management of pipeline variables"

What does this MR do?

Adds backend support for dynamic pipeline variable management.

EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4070 Frontend: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4110

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

Why was this MR needed?

N/A

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• N/A API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

Related #39118 (closed)