Hide common codec methods under helpers

The pending codec -> conversion split changes the signature of
Encode and Decode to be more complicated. Create a stub helper
with the exact semantics of today and do the simple mechanical
refactor here to reduce the cost of that change.

pkg/kubectl/rolling_updater_test.go

@@ -1099,7 +1099,7 @@ func readOrDie(t *testing.T, req *http.Request, codec runtime.Codec) runtime.Obj
 		t.Errorf("Error reading: %v", err)
 		t.FailNow()
 	}
-	obj, err := codec.Decode(data)
+	obj, err := runtime.Decode(codec, data)
 	if err != nil {
 		t.Errorf("error decoding: %v", err)
 		t.FailNow()