Extend API: Pipeline Schedule Variable

What does this MR do?

This MR extends API support for Pipeline Schedule Variable.

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

Is this easy to use?

Why was this MR needed?

API should be supported after the feature has been created.

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
• 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

What are the relevant issue numbers?

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