cmd/k8s-operator: move dnsrecords and nameserver into their own packages (#19696)

This commit moves the reconcilers for both the DNS nameserver and
DNSConfig custom resource into their own packages within
`k8s-operator/reconciler`

Closes: https://github.com/tailscale/corp/issues/37088

Signed-off-by: David Bond <davidsbond93@gmail.com>