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/applicationsets/platform.yaml

@@ -13,6 +13,7 @@ spec:
       - path: apps/overlays/*/artifactapi
       - path: apps/overlays/*/cattle-system
       - path: apps/overlays/*/certificates
+      - path: apps/overlays/*/cert-manager
       - path: apps/overlays/*/reflector-system
       - path: apps/overlays/*/reloader-system
       - path: apps/overlays/*/jfrog