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.

從 OAuth2 指南開始，檢查 GoogleAuthorizationCodeFlow 及其基底類別 AuthorizationCodeFlow，重點關注持久化為何需要 DataStoreFactory。將這項依賴與 Cloud Run 使用者可用的 DatastoreFactory 進行比較；完成的標準是就一個與儲存無關的擴充點達成共識並加以實作，同時測試顯示無需 App Engine 即可提供持久化。

由索引模型根據 Issue 內容生成。