feat: migrate externaldns from Terraform to ArgoCD

- Add externaldns base ArgoCD application with namespace and Vault integration
- Create externaldns overlay for au-syd1 with Helm chart configuration
- Update platform ApplicationSet to include externaldns deployment
- Configure external-dns v1.19.0 with RFC2136 provider for DNS updates
- Maintain one-to-one migration from Terraform configuration including TSIG secrets

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://kubernetes-sigs.github.io/external-dns/
   destinations:
     - namespace: '*-system'
       server: https://kubernetes.default.svc