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

Background

This issue has been raised when capturing the deltas in permissions between Developer and Maintainer. Maintainer has this permission, but Developer does not. Adding this as a customizable permission helps our customers lessen their reliance on the extremely privileged maintainer role, and gives them flexibility when creating new roles to give them only the permissions that they need.

Proposal

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