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/serviceaccount.yaml

@@ -0,0 +1,11 @@
+---
+apiVersion: v1
+kind: ServiceAccount
+metadata:
+  name: vault-issuer
+  namespace: cert-manager
+  labels:
+    app.kubernetes.io/name: "cert-manager-config"
+    app.kubernetes.io/instance: "cert-manager-config"
+    app.kubernetes.io/component: "vault-issuer"
+automountServiceAccountToken: true