google: Add support for 3-legged-OAuth using OAuth Client ID

Add OAuthClientTokenSource in google/google.go
Add DefaultAuthorizationHandler in authhandler.go

google/authhandler.go

@@ -0,0 +1,23 @@
+// 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 google
+
+import (
+	"fmt"
+)
+
+const DefaultState = "state"
+
+// DefaultAuthorizationHandler is a commandline-based auth handler
+// that prints the auth URL on the console and prompts the user to
+// authorize in the browser and paste the auth code back via stdin.
+// When using this auth handler, DefaultState must be used.
+func DefaultAuthorizationHandler(authCodeUrl string) (string, string, error) {
+	fmt.Printf("Go to the following link in your browser:\n\n   %s\n\n", authCodeUrl)
+	fmt.Println("Enter verification code: ")
+	var code string
+	fmt.Scanln(&code)
+	return code, DefaultState, nil
+}