authhandler: Add support for 3-legged-OAuth

Added authhandler.go, which implements a TokenSource
to support "three-legged OAuth 2.0" via a custom
AuthorizationHandler.

Added default_authhandler.go to provide a command line
implementation for AuthorizationHandler.

authhandler/default_authhandler.go

@@ -0,0 +1,42 @@
+// Copyright 2021 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 authhandler
+
+import (
+	"fmt"
+)
+
+// DefaultAuthorizationHandler returns a command line 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.
+//
+// Per OAuth protocol, a unique "state" string should be sent and verified
+// before exchanging auth code for OAuth token to prevent CSRF attacks.
+//
+// For convenience, this handler returns a pre-configured state
+// instead of asking the user to additionally paste the state from
+// the auth response. In order for this to work, the state
+// configured here must match the state used in authCodeURL.
+//
+// Usage example:
+//
+// state := uuid.New().String()
+// tokenSource:= authhandler.TokenSource(ctx, conf
+//     authhandler.DefaultAuthorizationHandler(state), state)
+// pubsubService, err := pubsub.NewService(ctx,
+//     option.WithTokenSource(tokenSource))
+func DefaultAuthorizationHandler(state string) AuthorizationHandler {
+	return func(authCodeURL string) (string, string, error) {
+		return defaultAuthorizationHandlerHelper(state, authCodeURL)
+	}
+}
+
+func defaultAuthorizationHandlerHelper(state string, authCodeURL string) (string, string, error) {
+	fmt.Printf("Go to the following link in your browser:\n\n   %s\n\n", authCodeURL)
+	fmt.Println("Enter authorization code: ")
+	var code string
+	fmt.Scanln(&code)
+	return code, state, nil
+}