feat: Ability to install cert-manager

Related issue: gitlab-org/gitlab#35664 (closed)

The values have been ported over from https://gitlab.com/gitlab-org/gitlab/tree/master/vendor/cert_manager

Overall approach:

• We first install cert-manager CRDs as instructed (https://raw.githubusercontent.com/jetstack/cert-manager/release-0.9/deploy/manifests/00-crds.yaml to match GitLab's version)
• Next we install cert-manager itself using Helmfile
  • Because of a bug in Helm, we need to wait a while here so that the admission.certmanager.k8s.io apiservice is ready.
• Finally we install the cluster issuer using yet another sub-helmfile.
  • This is optional, as we need user to provide a real email.