Deploy binddns-externaldns (RFC2136 dynamic cluster) (#222)

Part of the bind rollout split. **Merge #219 (bind-operator) first** — stacked on it; diff reduces to the binddns-externaldns files once #219 merges.

## Why
The external-dns tier (replaces 3x Puppet external-dns servers): an authoritative cluster whose zones accept RFC2136 TSIG updates from external-dns.

## Changes
- `apps/base/binddns-externaldns`: authoritative `BindCluster` (3 replicas, LoadBalancer/PureLB), `BindTSIGKey` for RFC2136, namespace
- au-syd1 `binddns-externaldns` overlay

## Deploy impact
Creates the `binddns-externaldns` StatefulSet + LoadBalancer once merged.

Reviewed-on: #222
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

apps/base/ns-externaldns/cluster.yaml

@@ -0,0 +1,39 @@
+---
+# external-dns tier (replaces the 3x Puppet external-dns servers). An ordinary
+# authoritative cluster; external-dns writes to its zones via RFC2136 because
+# those BindZones set dynamicUpdate (allow-update { key externaldns-key; }).
+apiVersion: bind.unkin.net/v1alpha1
+kind: BindCluster
+metadata:
+  name: externaldns
+  namespace: ns-externaldns
+spec:
+  mode: authoritative
+  replicas: 3
+  storageClassName: cephrbd-fast-delete
+  storageSize: 1Gi
+  service:
+    type: LoadBalancer
+    annotations:
+      purelb.io/service-group: common
+      purelb.io/addresses: 198.18.200.8
+      external-dns.alpha.kubernetes.io/hostname: ns-externaldns.k8s.syd1.au.unkin.net
+  resources:
+    requests:
+      cpu: 100m
+      memory: 128Mi
+    limits:
+      cpu: "1"
+      memory: 512Mi
+---
+# Catalog zone so the dynamic zones replicate onto the cluster's secondaries
+# (external-dns writes to the primary; secondaries IXFR the result).
+apiVersion: bind.unkin.net/v1alpha1
+kind: BindCatalogZone
+metadata:
+  name: externaldns-catalog
+  namespace: ns-externaldns
+spec:
+  clusterRef: externaldns
+  zoneName: catalog.externaldns.internal
+  transferKeyRef: externaldns-key