This introduces an Impersonate-Uid header to server side code.

UserInfo contains a uid field alongside groups, username and extra.
This change makes it possible to pass a UID through as an impersonation header like you
can with Impersonate-Group, Impersonate-User and Impersonate-Extra.

This PR contains:

* Changes to impersonation.go to parse the Impersonate-Uid header and authorize uid impersonation
* Unit tests for allowed and disallowed impersonation cases
* An integration test that creates a CertificateSigningRequest using impersonation,
  and ensures that the API server populates the correct impersonated spec.uid upon creation.

test/integration/authutil/authutil.go

@@ -0,0 +1,121 @@
+/*
+Copyright 2021 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.
+*/
+
+package authutil
+
+import (
+	"context"
+	"strings"
+	"testing"
+	"time"
+
+	authorizationv1 "k8s.io/api/authorization/v1"
+	rbacv1 "k8s.io/api/rbac/v1"
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	"k8s.io/apimachinery/pkg/runtime/schema"
+	"k8s.io/apimachinery/pkg/util/wait"
+	clientset "k8s.io/client-go/kubernetes"
+	authorizationv1client "k8s.io/client-go/kubernetes/typed/authorization/v1"
+)
+
+// WaitForNamedAuthorizationUpdate checks if the given user can perform the named verb and action on the named resource.
+// Copied from k8s.io/kubernetes/test/e2e/framework/auth.
+func WaitForNamedAuthorizationUpdate(t *testing.T, ctx context.Context, c authorizationv1client.SubjectAccessReviewsGetter, user, namespace, verb, resourceName string, resource schema.GroupResource, allowed bool) {
+	t.Helper()
+
+	review := &authorizationv1.SubjectAccessReview{
+		Spec: authorizationv1.SubjectAccessReviewSpec{
+			ResourceAttributes: &authorizationv1.ResourceAttributes{
+				Group:     resource.Group,
+				Verb:      verb,
+				Resource:  resource.Resource,
+				Namespace: namespace,
+				Name:      resourceName,
+			},
+			User: user,
+		},
+	}
+
+	if err := wait.Poll(200*time.Millisecond, wait.ForeverTestTimeout, func() (bool, error) {
+		response, err := c.SubjectAccessReviews().Create(ctx, review, metav1.CreateOptions{})
+		if err != nil {
+			return false, err
+		}
+		if response.Status.Allowed != allowed {
+			return false, nil
+		}
+		return true, nil
+	}); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func GrantUserAuthorization(t *testing.T, ctx context.Context, adminClient clientset.Interface, username string, rule rbacv1.PolicyRule) {
+	t.Helper()
+
+	cr, err := adminClient.RbacV1().ClusterRoles().Create(ctx, &rbacv1.ClusterRole{
+		ObjectMeta: metav1.ObjectMeta{GenerateName: strings.Replace(t.Name(), "/", "--", -1)},
+		Rules: []rbacv1.PolicyRule{
+			rule,
+		},
+	}, metav1.CreateOptions{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() {
+		_ = adminClient.RbacV1().ClusterRoles().Delete(ctx, cr.Name, metav1.DeleteOptions{})
+	})
+
+	crb, err := adminClient.RbacV1().ClusterRoleBindings().Create(ctx, &rbacv1.ClusterRoleBinding{
+		ObjectMeta: metav1.ObjectMeta{GenerateName: strings.Replace(t.Name(), "/", "--", -1)},
+		Subjects: []rbacv1.Subject{
+			{
+				Kind:      rbacv1.UserKind,
+				APIGroup:  rbacv1.GroupName,
+				Name:      username,
+				Namespace: "",
+			},
+		},
+		RoleRef: rbacv1.RoleRef{
+			APIGroup: rbacv1.GroupName,
+			Kind:     "ClusterRole",
+			Name:     cr.Name,
+		},
+	}, metav1.CreateOptions{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() {
+		_ = adminClient.RbacV1().ClusterRoleBindings().Delete(ctx, crb.Name, metav1.DeleteOptions{})
+	})
+
+	var resourceName string
+	if len(rule.ResourceNames) > 0 {
+		resourceName = rule.ResourceNames[0]
+	}
+
+	WaitForNamedAuthorizationUpdate(
+		t,
+		ctx,
+		adminClient.AuthorizationV1(),
+		username,
+		"",
+		rule.Verbs[0],
+		resourceName,
+		schema.GroupResource{Group: rule.APIGroups[0], Resource: rule.Resources[0]},
+		true,
+	)
+}