google: add utilities supporting upcoming oauth2 functionality

These are used to support some extended utilities to help with STS requests.

Change-Id: Iafc145b06ca42374cfc2ac6572762a50bcf560f2
GitHub-Last-Rev: 3085fe5703
GitHub-Pull-Request: golang/oauth2#439
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/259777
Trust: Cody Oss <codyoss@google.com>
Run-TryBot: Cody Oss <codyoss@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Tyler Bui-Palsulich <tbp@google.com>

google/internal/externalaccount/err_test.go

@@ -0,0 +1,19 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package externalaccount
+
+import "testing"
+
+func TestError(t *testing.T) {
+	e := Error{
+		"42",
+		"http:thisIsAPlaceholder",
+		"The Answer!",
+	}
+	want := "got error code 42 from http:thisIsAPlaceholder: The Answer!"
+	if got := e.Error(); got != want {
+		t.Errorf("Got error message %q; want %q", got, want)
+	}
+}