GoogleAuthorizationCodeFlow Won't Work With non-AppEngine

https://developers.google.com/api-client-library/java/google-api-java-client/oauth2

The `GoogleAuthorizationCodeFlow` class needs, for persistence, a `DataStoreFactory`. However, this appears to be accessible only with AppEngine because the Cloud-Run library provides instead a `DatastoreFactory` (small S).

This dependency goes down event into the `AuthorizationCodeFlow` base class making it (seemingly) impossible for me to create a custom derived class that manages the persistence on my terms.

It would be nice if the base class was independent of the storage type, using function calls that can be overridden.

Start with the OAuth2 guide and inspect GoogleAuthorizationCodeFlow and its AuthorizationCodeFlow base class, focusing on how DataStoreFactory is required for persistence. Compare that dependency with the DatastoreFactory available to Cloud Run users; done would mean agreeing on and implementing a storage-independent extension point, with tests showing persistence can be supplied without App Engine.

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