Create admin REST API

Create admin API for GitLab admins

I keep running into the situation, where for something as simple as cancelling pending jobs, I need to make couple of API calls which take several minutes just to gather the parameters I need to pass to the other API.

On the other hand, if I use the UI (Admin area ⏭ Jobs), this task is almost instantaneous.

I think admin API such as GET /admin/jobs would be a great addition to GitLab's current REST APIs and moreover would be greatly appreciated ⭐