API: Add the ability to fork to a specific namespace

Browser interface allows forking to an owned grup. This commit brings API up to speed by providing optional namespace parameter to fork API. This allows forking to users and groups under forker's control using their id or unique name.

Fixes #21591 (closed)

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 merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together