Add VolumeGroupSnapshot API definition, including CRD, client,

informer and lister generated code.
Update ./client/hack/README with instructions to update the client
directory.

client/informers/externalversions/volumegroupsnapshot/interface.go

@@ -0,0 +1,46 @@
+/*
+Copyright 2023 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+// Code generated by informer-gen. DO NOT EDIT.
+
+package volumegroupsnapshot
+
+import (
+	internalinterfaces "github.com/kubernetes-csi/external-snapshotter/client/v6/informers/externalversions/internalinterfaces"
+	v1alpha1 "github.com/kubernetes-csi/external-snapshotter/client/v6/informers/externalversions/volumegroupsnapshot/v1alpha1"
+)
+
+// Interface provides access to each of this group's versions.
+type Interface interface {
+	// V1alpha1 provides access to shared informers for resources in V1alpha1.
+	V1alpha1() v1alpha1.Interface
+}
+
+type group struct {
+	factory          internalinterfaces.SharedInformerFactory
+	namespace        string
+	tweakListOptions internalinterfaces.TweakListOptionsFunc
+}
+
+// New returns a new Interface.
+func New(f internalinterfaces.SharedInformerFactory, namespace string, tweakListOptions internalinterfaces.TweakListOptionsFunc) Interface {
+	return &group{factory: f, namespace: namespace, tweakListOptions: tweakListOptions}
+}
+
+// V1alpha1 returns a new v1alpha1.Interface.
+func (g *group) V1alpha1() v1alpha1.Interface {
+	return v1alpha1.New(g.factory, g.namespace, g.tweakListOptions)
+}