Extend API: Group Secret Variable

What does this MR do?

This MR adds API support for Group-level Secret Variable. Group-level Secret Variable was introduced at %9.4.

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

This is the first API for Group-level settings of CI.

Why was this MR needed?

We should support API when introduced a new feature.

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
  • 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 #34519 (closed)