cli: create backups for CRDs and their resources

These backups could be used in case an upgrade
misbehaves after helm declared it as successful.
The manual backups are required as helm-rollback
won't touch custom resources and changes to CRDs
delete resources of the old version.

internal/kubernetes/kubectl/restclient.go

@@ -0,0 +1,70 @@
+/*
+Copyright (c) Edgeless Systems GmbH
+
+SPDX-License-Identifier: AGPL-3.0-only
+*/
+
+package kubectl
+
+import (
+	"k8s.io/apimachinery/pkg/api/meta"
+	"k8s.io/client-go/discovery"
+	"k8s.io/client-go/discovery/cached/memory"
+	"k8s.io/client-go/rest"
+	"k8s.io/client-go/restmapper"
+	"k8s.io/client-go/tools/clientcmd"
+)
+
+// restClientGetter implements k8s.io/cli-runtime/pkg/resource.RESTClientGetter.
+type restClientGetter struct {
+	clientconfig clientcmd.ClientConfig
+}
+
+// newRESTClientGetter creates a new restClientGetter using a kubeconfig.
+func newRESTClientGetter(kubeconfig []byte) (*restClientGetter, error) {
+	clientconfig, err := clientcmd.NewClientConfigFromBytes(kubeconfig)
+	if err != nil {
+		return nil, err
+	}
+
+	rawconfig, err := clientconfig.RawConfig()
+	if err != nil {
+		return nil, err
+	}
+
+	clientconfig = clientcmd.NewDefaultClientConfig(rawconfig, &clientcmd.ConfigOverrides{})
+
+	return &restClientGetter{clientconfig}, nil
+}
+
+// ToRESTConfig returns k8s REST client config.
+func (r *restClientGetter) ToRESTConfig() (*rest.Config, error) {
+	return r.clientconfig.ClientConfig()
+}
+
+// ToDiscoveryClient creates new k8s discovery client from restClientGetter.
+func (r *restClientGetter) ToDiscoveryClient() (discovery.CachedDiscoveryInterface, error) {
+	restconfig, err := r.clientconfig.ClientConfig()
+	if err != nil {
+		return nil, err
+	}
+	dc, err := discovery.NewDiscoveryClientForConfig(restconfig)
+	if err != nil {
+		return nil, err
+	}
+	return memory.NewMemCacheClient(dc), nil
+}
+
+// ToRESTMapper creates new k8s RESTMapper from restClientGetter.
+func (r *restClientGetter) ToRESTMapper() (meta.RESTMapper, error) {
+	dc, err := r.ToDiscoveryClient()
+	if err != nil {
+		return nil, err
+	}
+	return restmapper.NewDeferredDiscoveryRESTMapper(dc), nil
+}
+
+// ToRawKubeConfigLoader returns the inner k8s ClientConfig.
+func (r *restClientGetter) ToRawKubeConfigLoader() clientcmd.ClientConfig {
+	return r.clientconfig
+}