Support overriding the target of endpoints per-address family

[starcraft66]

What would you like to be added: Support for defining the target ip address of a certain address family while using the discovered ip address of the other family.

Why is this needed:

My dual-stack kubernetes cluster (and I suspect many others) is located behind an IPv4 NAT, but has native IPv6 access. This means that services of type LoadBalancer are assigned ExtenalIPs (by cilium lb-ipam in my case) in both the IPv4 RFC1918 range and a GUA address pool dedicated to my cluster.

External IPv4 connections are NATed on a router in front of the kubernetes cluster and kubernetes has no idea what the outside address of this NAT is, but I can manually specify it via the external-dns.alpha.kubernetes.io/target annotation. The problem is that if I do this, the discovered IPv6 record will not be published because I just overrode it with only an IPv4 address.

This leaves me in 3 situations, not of them being optimal IMO:

1. I don't use any annotation, resulting in external-dns publishing my service's RFC1918 address in the A record and the IPv6 GUA address in the AAAA record. Result: IPv6 connectivity theoretically works fine but IPv4 connectivity will resolve to an address people outside the LAN can't reach.

2. I use the external-dns.alpha.kubernetes.io/target annotation set to an IPv4 address to publish an A record with the external IP of my NAT. Result: IPv4 connectivity works fine but no AAAA record will be published so IPv6 is broken.

3. I use the external-dns.alpha.kubernetes.io/target to set both and IPv4 and IPv6 address, comma-separated. Result: IPv4 and IPv6 connectivity work fine, but I have to manually define the ExternalIP on my services to avoid desyncs and I lose the niceties of having IP addresses dynamically allocated by my k8s loadbalancer.

It would be nice to either have extra annotations to support this use case or a cli flag that would allow you to keep discovering addresses of a different family if external-dns.alpha.kubernetes.io/target only contains addresses of a certain family.