[WIP] Make possible to mark issue as confidential via API

What does this MR do?

This MR makes possible to create confidential issue via API or mark an existing issue as an confidential via API.

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

I don't think so.

Why was this MR needed?

We already have API to manage issues. We can create or update the issue via API. But we are unable to mark this issue as a confidential (with both create and edit endpoints).

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together