Implement session affinity in the CoreDNS plugin

[tpantelis]

We now propagate the session affinity fields from the exported Service to the aggregated ServiceImport. Currently this is to support MCS compliance and other integrations, eg Istio, that may utilize it. We should also consider implementing it in the CoreDNS plugin wrt service ClusterIPs, ie keep a client, identified by its IP, routed to the same cluster IP after the first request instead of always round-robin. We'd also need to determine what the behavior should be if the constituent exported services conflict wrt the SessionAffinity field.