kubernetes: add multicluster support (#7266)

* kubernetes: add multicluster support

Add multicluster support via Multi-Cluster Services API (MCS-API) via a
new option `multiclusterZones` in the kubernetes plugin.

When some multicluster zones are passed to the kubernetes plugin, it
will start watching the ServiceImport objects and its associated
EndpointSlices.

Signed-off-by: Arthur Outhenin-Chalandre <arthur@cri.epita.fr>

* kubernetes: implement xfr support for multicluster zones

Signed-off-by: Arthur Outhenin-Chalandre <arthur@cri.epita.fr>

---------

Signed-off-by: Arthur Outhenin-Chalandre <arthur@cri.epita.fr>

plugin/kubernetes/metadata_test.go

@@ -2,6 +2,7 @@ package kubernetes
 
 import (
 	"context"
+	"strings"
 	"testing"
 
 	"github.com/coredns/coredns/plugin/metadata"
@@ -81,6 +82,19 @@ var metadataCases = []struct {
 			"kubernetes/service":   "s",
 		},
 	},
+	{
+		Qname: "ep.c1.s.ns.svc.clusterset.local.", Qtype: dns.TypeA,
+		RemoteIP: "10.10.10.10",
+		Md: map[string]string{
+			"kubernetes/cluster":   "c1",
+			"kubernetes/endpoint":  "ep",
+			"kubernetes/kind":      "svc",
+			"kubernetes/namespace": "ns",
+			"kubernetes/port-name": "",
+			"kubernetes/protocol":  "",
+			"kubernetes/service":   "s",
+		},
+	},
 	{
 		Qname: "example.com.", Qtype: dns.TypeA,
 		RemoteIP: "10.10.10.10",
@@ -103,14 +117,19 @@ func mapsDiffer(a, b map[string]string) bool {
 }
 
 func TestMetadata(t *testing.T) {
-	k := New([]string{"cluster.local."})
+	k := New([]string{"cluster.local.", "clusterset.local."})
+	k.opts.multiclusterZones = []string{"clusterset.local."}
 	k.APIConn = &APIConnServeTest{}
 
 	for i, tc := range metadataCases {
 		ctx := metadata.ContextWithMetadata(context.Background())
+		zone := "."
+		if strings.Contains(tc.Qname, "clusterset.local") {
+			zone = "clusterset.local."
+		}
 		state := request.Request{
 			Req:  &dns.Msg{Question: []dns.Question{{Name: tc.Qname, Qtype: tc.Qtype}}},
-			Zone: ".",
+			Zone: zone,
 			W:    &test.ResponseWriter{RemoteIP: tc.RemoteIP},
 		}