Remove client pkg from kubectl pkg (#638)

The nested client pkg was necessary to implement a generator pattern.
The generator was necessary as the Kubewrapper type
expects a k8sapi.Client object during instantiation.
However, the required kubeconfig is not ready during Kubewrapper creation.
This patch relies on an Initialize function to set the Kubeconfig
and hands over an empty struct during Kubewrapper creation.
This allows us to remove the extra Client abstraction.

bootstrapper/internal/kubernetes/k8sapi/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
+}