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>
