Make wrapping a client transport more pleasant

Properly wrapping a transport can be tricky. Make the normal case
(adding a non-nil transport wrapper to a config) easier with a helper.
Also enforce a rough ordering, which in the future we can use to
simplify the WrapTransport mechanism down into an array of functions
we execute in order and avoid wrapping altogether.

Kubernetes-commit: 1f590e697ef64812620c787720b4b5942027e4a1

rest/config_test.go

@@ -27,12 +27,13 @@ import (
 	"strings"
 	"testing"
 
-	"k8s.io/api/core/v1"
+	v1 "k8s.io/api/core/v1"
 	"k8s.io/apimachinery/pkg/runtime"
 	"k8s.io/apimachinery/pkg/runtime/schema"
 	"k8s.io/apimachinery/pkg/util/diff"
 	"k8s.io/client-go/kubernetes/scheme"
 	clientcmdapi "k8s.io/client-go/tools/clientcmd/api"
+	"k8s.io/client-go/transport"
 	"k8s.io/client-go/util/flowcontrol"
 
 	fuzz "github.com/google/gofuzz"
@@ -236,6 +237,9 @@ func TestAnonymousConfig(t *testing.T) {
 		func(fn *func(http.RoundTripper) http.RoundTripper, f fuzz.Continue) {
 			*fn = fakeWrapperFunc
 		},
+		func(fn *transport.WrapperFunc, f fuzz.Continue) {
+			*fn = fakeWrapperFunc
+		},
 		func(r *runtime.NegotiatedSerializer, f fuzz.Continue) {
 			serializer := &fakeNegotiatedSerializer{}
 			f.Fuzz(serializer)
@@ -316,6 +320,9 @@ func TestCopyConfig(t *testing.T) {
 		func(fn *func(http.RoundTripper) http.RoundTripper, f fuzz.Continue) {
 			*fn = fakeWrapperFunc
 		},
+		func(fn *transport.WrapperFunc, f fuzz.Continue) {
+			*fn = fakeWrapperFunc
+		},
 		func(r *runtime.NegotiatedSerializer, f fuzz.Continue) {
 			serializer := &fakeNegotiatedSerializer{}
 			f.Fuzz(serializer)