Add "manage group level CI/CD variables" as a customizable permission

Background

This issue has been raised by customers who are interested in customizable permissions via this feedback issue. Previously this feedback was lumped in with Add "manage project level CI/CD variables" as a... (#421795 - closed), but I am creating this issue to separate out project and group.

Proposal

Add "manage group level CI/CD variables" as a customizable permission using the customizable roles framework.