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

argocd/applicationsets/platform.yaml

@@ -14,6 +14,7 @@ spec:
       - path: apps/overlays/*/cattle-system
       - path: apps/overlays/*/certificates
       - path: apps/overlays/*/cert-manager
+      - path: apps/overlays/*/externaldns
       - path: apps/overlays/*/reflector-system
       - path: apps/overlays/*/reloader-system
       - path: apps/overlays/*/jfrog

argocd/projects/platform.yaml

@@ -12,6 +12,7 @@ spec:
     - oci://ghcr.io/woodpecker-ci/helm/woodpecker
     - https://releases.rancher.com/server-charts/stable
     - https://charts.jetstack.io
+    - https://kubernetes-sigs.github.io/external-dns/
   destinations:
     - namespace: '*-system'
       server: https://kubernetes.default.svc
@@ -21,6 +22,8 @@ spec:
       server: https://kubernetes.default.svc
     - namespace: 'certificates'
       server: https://kubernetes.default.svc
+    - namespace: 'externaldns'
+      server: https://kubernetes.default.svc
     - namespace: 'jfrog'
       server: https://kubernetes.default.svc
     - namespace: 'puppet'