Provide a constant for authorization URI for web and installed app flows.

[vtsao]

When using this library to generate a refresh token for web/installed app flows, there isn't a constant for the authorization URI:
https://accounts.google.com/o/oauth2/v2/auth

https://developers.google.com/identity/protocols/OAuth2WebServer#redirecting

Does it make sense to put this somewhere in this library? Maybe in UserRefreshCredentials?