WIP: DRY and refactor CI/CD Variables

What does this MR do?

This MR refactors how we handle CI/CD Variables collections.

Why was this MR needed?

We have a lot of duplication regarding CI/CD variables what causes some problems and productivity loss.

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Has been reviewed by Backend

What are the relevant issue numbers?

Related gitlab-org/gitlab-ce#33042