Authoritative zones: accept puppet client dynamic updates (#244)

Enables per-host RFC2136 updates from puppet (puppet-prod #475 profiles::dns::updater) to the bind-authoritative zones, via the .9 write endpoint.

## Changes
- add **client-update** BindTSIGKey (clusterRef bind-authoritative; operator generates the material into Secret client-update-tsig)
- set `dynamicUpdate: true` + `updateKeyRef: client-update` on all **18** authoritative zones → the operator renders `allow-update { key "client-update"; }`

## Key bridge (manual, per the TSIG plan)
The operator generates the client-update key value; it must reach puppet eyaml (`profiles::dns::updater::key_secret`) for clients to authenticate — until the planned Vault-sync/secret-reflection operator features exist. Get it with:
`kubectl -n bind-internal get secret client-update-tsig -o jsonpath='{.data.secret}' | base64 -d`

## Validated
kustomize build + kubeconform.

Reviewed-on: #244
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

apps/base/bind-internal/authoritative/tsigkey.yaml

@@ -9,3 +9,16 @@ metadata:
 spec:
   clusterRef: bind-authoritative
   algorithm: hmac-sha256
+---
+# Client-update key: puppet clients (profiles::dns::updater) nsupdate their own
+# records to the authoritative zones with this key. Operator generates the
+# material into Secret client-update-tsig; the same value must reach puppet
+# eyaml (or the planned Vault-sync bridge) for clients to authenticate.
+apiVersion: bind.unkin.net/v1alpha1
+kind: BindTSIGKey
+metadata:
+  name: client-update
+  namespace: bind-internal
+spec:
+  clusterRef: bind-authoritative
+  algorithm: hmac-sha256