WIP: BE: Adding variables to pipelines schedules

What does this MR do?

This MR

• adds a functionality of adding variables to scheduled pipelines
• covers only BE parts. i.e. Model, DB, Controller, etc
• joins FE code at https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12372

The specification overview has been described at https://gitlab.com/gitlab-org/gitlab-ce/issues/32568#note_32683716.

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

We use HasVariable to generalize the codebase related to Project secret variables. Need to check that we don't break the existed functionality.

Why was this MR needed?

Scheduled pipelines will be more useful with Variables. Originally issued at https://gitlab.com/gitlab-org/gitlab-ce/issues/32568

Screenshots (if relevant)

This should be done in FE part. Originally proposed at https://gitlab.com/gitlab-org/gitlab-ce/issues/32568#proposal.

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?

• #32568 (closed)