For custom AuthenticatorDelegate implementations it may be very weird to
be required to return an additional character after the code from
present_user_url.
One may find himself chasing the bug which is bad UX for the user.
Improve this behavior and therefore introduce a breaking change that
will hopefully not break existing implementations nevertheless.
When implementing AuthenticatorDelegate one might want to change the
redirect_uri to use an authorized domain or just change the way
InstalledFlow works.
Add a redirect_uri method to AuthenticatorDelegate that may be
implemented to define a custom redirect uri if needed.
I mainly resolved some circular dependencies that had crept in, and
moved code around. I renamed helper.rs because that was not really an
appropriate name anymore, and moved the delegate code into a new module.
The "Installed App Flow" requires user interaction; we first generate a
URL that the user has to navigate to. Then, the user either pastes a
displayed code into the app being authorized, or the OAuth provider
redirects the user's browser to a webserver that is running on
localhost. This webserver is provided by the library and the flow should
work automatically. Extensive documentation can be found here:
https://developers.google.com/identity/protocols/OAuth2InstalledApp
An example for the InstalledFlow with the Drive API is here:
https://gist.github.com/dermesser/8c915ec4c88ee8e8927e7d40b276ca52
