Add open_issues}/%closed_issues for badges

Similar to #56995 (moved) it'd be nice to have an open_issues/closed_issues tag for badges, especially if there's a way to indicate issues for a specific milestone.

Something like this:

https://gitlab.com/%{project_path}/%{project_id}/badges/open_issues.svg for overall

https://gitlab.com/%{project_path}/%{project_id}/badges/1.0.0/open_issues.svg for issues tagged for a specific milestone