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/object/multicluster_endpoint.go

@@ -0,0 +1,64 @@
+package object
+
+import (
+	"maps"
+
+	meta "k8s.io/apimachinery/pkg/apis/meta/v1"
+	"k8s.io/apimachinery/pkg/runtime"
+	mcs "sigs.k8s.io/mcs-api/pkg/apis/v1alpha1"
+)
+
+// Endpoints is a stripped down api.Endpoints with only the items we need for CoreDNS.
+type MultiClusterEndpoints struct {
+	Endpoints
+	ClusterId string
+	*Empty
+}
+
+// MultiClusterEndpointsKey returns a string using for the index.
+func MultiClusterEndpointsKey(name, namespace string) string { return name + "." + namespace }
+
+// EndpointSliceToEndpoints converts a *discovery.EndpointSlice to a *Endpoints.
+func EndpointSliceToMultiClusterEndpoints(obj meta.Object) (meta.Object, error) {
+	labels := maps.Clone(obj.GetLabels())
+	ends, err := EndpointSliceToEndpoints(obj)
+	if err != nil {
+		return nil, err
+	}
+	e := &MultiClusterEndpoints{
+		Endpoints: *ends.(*Endpoints),
+		ClusterId: labels[mcs.LabelSourceCluster],
+	}
+	e.Index = MultiClusterEndpointsKey(labels[mcs.LabelServiceName], ends.GetNamespace())
+
+	return e, nil
+}
+
+var _ runtime.Object = &Endpoints{}
+
+// DeepCopyObject implements the ObjectKind interface.
+func (e *MultiClusterEndpoints) DeepCopyObject() runtime.Object {
+	e1 := &MultiClusterEndpoints{
+		ClusterId: e.ClusterId,
+		Endpoints: *e.Endpoints.DeepCopyObject().(*Endpoints),
+	}
+	return e1
+}
+
+// GetNamespace implements the metav1.Object interface.
+func (e *MultiClusterEndpoints) GetNamespace() string { return e.Endpoints.GetNamespace() }
+
+// SetNamespace implements the metav1.Object interface.
+func (e *MultiClusterEndpoints) SetNamespace(namespace string) {}
+
+// GetName implements the metav1.Object interface.
+func (e *MultiClusterEndpoints) GetName() string { return e.Endpoints.GetName() }
+
+// SetName implements the metav1.Object interface.
+func (e *MultiClusterEndpoints) SetName(name string) {}
+
+// GetResourceVersion implements the metav1.Object interface.
+func (e *MultiClusterEndpoints) GetResourceVersion() string { return e.Endpoints.GetResourceVersion() }
+
+// SetResourceVersion implements the metav1.Object interface.
+func (e *MultiClusterEndpoints) SetResourceVersion(version string) {}