feat: migrate externaldns from Terraform to ArgoCD (#43)

- 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

Reviewed-on: #43

apps/overlays/au-syd1/externaldns/kustomization.yaml

@@ -0,0 +1,14 @@
+---
+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+
+resources:
+  - ../../../base/externaldns
+
+helmCharts:
+  - name: external-dns
+    repo: https://kubernetes-sigs.github.io/external-dns/
+    version: "1.19.0"
+    releaseName: externaldns
+    namespace: externaldns
+    valuesFile: values.yaml