Support a non-default cluster domain

[andyasp]

As mentioned in #258 (review), when constructing pod endpoints the cluster domain is hardcoded to the default cluster.local. It could be nice to support a non-default value.

Code reference:

func StatefulSetPodFQDN(namespace, statefulSetName string, ordinal int, serviceName string) string {
	// The DNS entry for a pod of a stateful set is
	// $(statefulset name)-(ordinal).$(service name).$(namespace).svc.cluster.local
	// https://kubernetes.io/docs/concepts/workloads/controllers/statefulset/#stable-network-id
	//
	// Ending that with a trailing "." to signify an absolute domain name
	// https://datatracker.ietf.org/doc/html/rfc1034#section-3.1
	return fmt.Sprintf("%v-%v.%v.%v.svc.cluster.local.",
		statefulSetName,
		ordinal,
		serviceName,
		namespace,
	)
}

Kubernetes documentation: https://kubernetes.io/docs/concepts/workloads/controllers/statefulset/#stable-network-id