cmd/k8s-operator: support imagePullSecrets on DNSConfig nameserver

The nameserver Deployment created for a DNSConfig was the only
operator-managed workload with no way to configure imagePullSecrets, so its
pods could not pull the nameserver image from a private registry.

Adds imagePullSecrets to NameserverPod and thread it through to the nameserver
Deployment pod spec.

Updates #16772

Signed-off-by: chaosinthecrd <tom@tmlabs.co.uk>