Consolidate BIND DNS into one bind-internal namespace

Reshapes the three DNS tiers from separate ns-* namespaces into a single
bind-internal namespace and renames the StatefulSets, and scopes the TSIG
keys to their cluster (needs the clusterRef field from operator v0.1.3).

- move the 3 clusters + zones + keys into apps/base/bind-internal:
  BindCluster names bind-authoritative / bind-resolvers / bind-externaldns
  (= StatefulSet names), LBs kept on .6/.7/.8, external-dns hostnames renamed
- add clusterRef to the transfer-key (bind-authoritative) and externaldns-key
  (bind-externaldns) TSIG keys so they no longer leak across clusters
- remove the old ns-auth / ns-resolver / ns-externaldns apps
- ApplicationSet + AppProject: replace the three ns-* entries with bind-internal
- bump bind-system operator to v0.1.3 (CRD install link + image) so the CRDs
  understand the new clusterRef field
- operator stays in bind-system (unchanged)

apps/base/bind-internal/kustomization.yaml

@@ -0,0 +1,9 @@
+---
+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+
+resources:
+  - namespace.yaml
+  - authoritative
+  - resolvers
+  - externaldns