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/serviceimport.go

@@ -0,0 +1,95 @@
+package object
+
+import (
+	"fmt"
+
+	meta "k8s.io/apimachinery/pkg/apis/meta/v1"
+	"k8s.io/apimachinery/pkg/runtime"
+	mcs "sigs.k8s.io/mcs-api/pkg/apis/v1alpha1"
+)
+
+// ServiceImport is a stripped down api.ServiceImport with only the items we need for CoreDNS.
+type ServiceImport struct {
+	Version    string
+	Name       string
+	Namespace  string
+	Index      string
+	ClusterIPs []string
+	Type       mcs.ServiceImportType
+	Ports      []mcs.ServicePort
+
+	*Empty
+}
+
+// ServiceImportKey returns a string using for the index.
+func ServiceImportKey(name, namespace string) string { return name + "." + namespace }
+
+// ToServiceImport converts an v1alpha1.ServiceImport to a *ServiceImport.
+func ToServiceImport(obj meta.Object) (meta.Object, error) {
+	svc, ok := obj.(*mcs.ServiceImport)
+
+	if !ok {
+		return nil, fmt.Errorf("unexpected object %v", obj)
+	}
+	s := &ServiceImport{
+		Version:   svc.GetResourceVersion(),
+		Name:      svc.GetName(),
+		Namespace: svc.GetNamespace(),
+		Index:     ServiceImportKey(svc.GetName(), svc.GetNamespace()),
+		Type:      svc.Spec.Type,
+	}
+
+	if len(svc.Spec.IPs) > 0 {
+		s.ClusterIPs = make([]string, len(svc.Spec.IPs))
+		copy(s.ClusterIPs, svc.Spec.IPs)
+	}
+
+	if len(svc.Spec.Ports) > 0 {
+		s.Ports = make([]mcs.ServicePort, len(svc.Spec.Ports))
+		copy(s.Ports, svc.Spec.Ports)
+	}
+
+	*svc = mcs.ServiceImport{}
+	return s, nil
+}
+
+var _ runtime.Object = &ServiceImport{}
+
+// Headless returns true if the service is headless
+func (s *ServiceImport) Headless() bool {
+	return s.Type == mcs.Headless
+}
+
+// DeepCopyObject implements the ObjectKind interface.
+func (s *ServiceImport) DeepCopyObject() runtime.Object {
+	s1 := &ServiceImport{
+		Version:    s.Version,
+		Name:       s.Name,
+		Namespace:  s.Namespace,
+		Index:      s.Index,
+		Type:       s.Type,
+		ClusterIPs: make([]string, len(s.ClusterIPs)),
+		Ports:      make([]mcs.ServicePort, len(s.Ports)),
+	}
+	copy(s1.ClusterIPs, s.ClusterIPs)
+	copy(s1.Ports, s.Ports)
+	return s1
+}
+
+// GetNamespace implements the metav1.Object interface.
+func (s *ServiceImport) GetNamespace() string { return s.Namespace }
+
+// SetNamespace implements the metav1.Object interface.
+func (s *ServiceImport) SetNamespace(namespace string) {}
+
+// GetName implements the metav1.Object interface.
+func (s *ServiceImport) GetName() string { return s.Name }
+
+// SetName implements the metav1.Object interface.
+func (s *ServiceImport) SetName(name string) {}
+
+// GetResourceVersion implements the metav1.Object interface.
+func (s *ServiceImport) GetResourceVersion() string { return s.Version }
+
+// SetResourceVersion implements the metav1.Object interface.
+func (s *ServiceImport) SetResourceVersion(version string) {}