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/base/externaldns/vaultstaticsecret.yaml

@@ -0,0 +1,18 @@
+---
+apiVersion: secrets.hashicorp.com/v1beta1
+kind: VaultStaticSecret
+metadata:
+  name: externaldns-tsig
+  namespace: externaldns
+spec:
+  vaultAuthRef: default
+  mount: kv
+  type: kv-v2
+  path: service/kubernetes/au/syd1/externaldns/tsig
+  refreshAfter: 5m
+  destination:
+    name: externaldns-tsig
+    create: true
+  rolloutRestartTargets:
+    - kind: Deployment
+      name: externaldns