Implement `HasVariable` concern

What does this MR do?

This MR adds HasVariable concern for for sharing Ci::Variable core implementation with Group/Project/PipelineSchedule Variables

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

Need to check that we don't break the old Ci::Variable functionality.

Why was this MR needed?

This MR is useful for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12341 and https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11078

Screenshots (if relevant)

N/A

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #34314 (closed)

/cc @godfat @ayufan @zj