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

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

@@ -0,0 +1,51 @@
+# Service account configuration
+serviceAccount:
+  create: true
+  name: externaldns
+  annotations: {}
+
+# Provider configuration - using new format
+provider:
+  name: rfc2136
+
+# Domain filtering
+domainFilters:
+  - "k8s.syd1.au.unkin.net"
+  - "200.18.198.in-addr.arpa"
+
+# TXT registry configuration
+txtOwnerId: "k8s"
+registry: "txt"
+
+# Enable deletion of records for dedicated DNS server
+policy: "sync"
+
+# Keep default sources
+sources:
+  - service
+  - ingress
+
+# Environment variables for TSIG secret and algorithm from Vault
+env:
+  - name: EXTERNAL_DNS_RFC2136_TSIG_SECRET
+    valueFrom:
+      secretKeyRef:
+        name: externaldns-tsig
+        key: secret
+  - name: EXTERNAL_DNS_RFC2136_TSIG_ALGORITHM
+    valueFrom:
+      secretKeyRef:
+        name: externaldns-tsig
+        key: algorithm
+
+# RFC2136 configuration as arguments
+extraArgs:
+  - --rfc2136-host=ausyd1nxvm2127.main.unkin.net
+  - --rfc2136-port=53
+  - --rfc2136-zone=k8s.syd1.au.unkin.net
+  - --rfc2136-zone=200.18.198.in-addr.arpa
+  - --rfc2136-tsig-keyname=externaldns-key
+  - --rfc2136-tsig-secret-alg=$(EXTERNAL_DNS_RFC2136_TSIG_ALGORITHM)
+  - --rfc2136-tsig-axfr
+  - --rfc2136-tsig-secret=$(EXTERNAL_DNS_RFC2136_TSIG_SECRET)
+  - --ingress-class=nginx