Port IAM policy configuration from gitlab-project-factory
As noted in https://gitlab.developers.cam.ac.uk/uis/devops/infra/gitlab-project-factory/-/merge_requests/51#note_565408, enable management of IAM policy for secrets.
As noted in https://gitlab.developers.cam.ac.uk/uis/devops/infra/gitlab-project-factory/-/merge_requests/51#note_565408, enable management of IAM policy for secrets.