feat: migrate cert-manager from Terraform to ArgoCD (#42)

- 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

Reviewed-on: #42

apps/base/cert-manager/clusterrolebinding.yaml

@@ -0,0 +1,16 @@
+---
+apiVersion: rbac.authorization.k8s.io/v1
+kind: ClusterRoleBinding
+metadata:
+  name: cert-manager-vault-token-creator
+  labels:
+    app.kubernetes.io/name: "cert-manager-config"
+    app.kubernetes.io/instance: "cert-manager-config"
+roleRef:
+  apiGroup: rbac.authorization.k8s.io
+  kind: ClusterRole
+  name: cert-manager-vault-token-creator
+subjects:
+- kind: ServiceAccount
+  name: cert-manager
+  namespace: cert-manager