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

apps/base/cert-manager/namespace.yaml

@@ -0,0 +1,5 @@
+---
+apiVersion: v1
+kind: Namespace
+metadata:
+  name: cert-manager