Add support for GitLab group variables

[robertohueso]

Summary

This PR refactors pkg/cosign/git/gitlab/gitlab.go to make it work with both project variables and group variables. This is mainly used when running

cosign generate-key-pair gitlab://<path>

This solves #2914 for GitLab.

Release Note

• GitLab go-cloud style URIs now work both projects and groups.

Documentation

This probably does not require any change in docs apart from the ones included in this PR.