Add variables to pipelines schedules

What does this MR do?

This MR is an integrated branch from FE and BE changes for https://gitlab.com/gitlab-org/gitlab-ce/issues/32568.

EE MR(new), https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2383 EE MR(old), https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2347

Mainly this MR is expecting

• Writing feature specs
• Writing docs
• Resolve all temporary codes which expected to be modified by another side
• Approved by FE and BE end bosses

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 -> https://gitlab.com/gitlab-org/gitlab-ce/issues/34518
• 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?

• Issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/32568
• FE MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12328
• BE MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12341

Close #32568 (closed)