Add API support for pipeline schedule

What does this MR do?

This MR adds API support for pipeline schedule. In this first iteration, we will prepare elementary endpoints which were proposed at https://gitlab.com/gitlab-org/gitlab-ce/issues/30892#note_29288448.

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

There are no critical changes.

Why was this MR needed?

We should support API with new functionality.

Screenshots (if relevant)

No changes for UI

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 #30892 (closed)

/cc @zj