feat: migrate cert-manager from Terraform to ArgoCD

- Add cert-manager base ArgoCD application with namespace, RBAC resources
- Create cert-manager overlay for au-syd1 with Helm chart configuration
- Update platform ApplicationSet to include cert-manager deployment
- Configure cert-manager v1.19.2 with jetstack Helm repository
- Maintain one-to-one migration from Terraform configuration

argocd/projects/platform.yaml

@@ -10,6 +10,7 @@ spec:
     - https://git.unkin.net/unkin/argocd-apps
     - oci://ghcr.io/emberstack/helm-charts
     - oci://ghcr.io/woodpecker-ci/helm/woodpecker
+    - https://charts.jetstack.io
   destinations:
     - namespace: '*-system'
       server: https://kubernetes.default.svc