Extend API: Pipeline Schedule Variable (Old. With Grape:Json. Fully integrated with parent)

What does this MR do?

This MR adds an API support for Pipeline schedule variables.

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

Does the validation work?

Why was this MR needed?

When a feature was created, the corresponded API should be updated too.

Screenshots (if relevant)

No. This is API extension.

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?

Close #34518 (closed)