Implement `GET /hooks/:id` API

The System Hooks API currently doesn't support getting a single system hook. Having this implemented will enable the terraform-gitlab-provider to properly implement a resource for system hooks without relying on and filtering in /hooks.

I'll created a MR in a sec. edit: !81595 (merged)

/cc @nagyv-gitlab @nmezzopera can you please help me triage this, thanks 🎉