[external-dns] Private hosts in public DNZ zone

Hi,
I'm trying to add internal ingress using instruction https://learn.microsoft.com/en-us/azure/aks/create-nginx-ingress-private-controller.
Current controller implementation does not provide configuration options for external-dns controller.

# Issue
All hosts from Private DNS Zone `private.example.com` created in DNS zone `example.com` also.
This makes impossible to use operator for split-horizon/split-view DNS completely.

# Proposal
Add configuration options to `NginxIngressController` for external-dns controller to filter watched ingresses:
- annotation-filter
- label-filter
- ingress-class (available from [v0.13.5](https://github.com/kubernetes-sigs/external-dns/blob/v0.13.5/pkg/apis/externaldns/types.go))

Start at NginxIngressController and inspect how its external-dns controller configuration is built; use the external-dns v0.13.5 API linked in the issue as the reference for annotation-filter, label-filter, and ingress-class. Done means these options can filter watched ingresses so private hosts are not published in the public DNS zone, with behavior verified against the split-horizon scenario.

Written by the indexing model from the issue text.