Make FAB auth manager login process compatible with Airflow 3 UI

[vincbeck]

The plan to support FAB auth manager in Airflow 3 is:

• Create a trimmed version of the Airflow 2 Flask application in FAB provider. This is done ✅
• Embed this Flask application in fastapi application using WSGIMiddleware when FAB auth manager is the auth manager configured in the Airflow environment. This is done ✅
• Update the FAB provider Flask application login process to be compatible with Airflow 3 UI. This is this PR
• Update get_url_login method in FAB auth manager to return the appropriate login URL so that the fastapi application can retrieve this value and pass it to the front-end. On the front-end, if the user is not authenticated, it will redirect the user to this URL. It will be done in a future PR.

Testing

You can test this PR by going to <endpoint>/auth/loginand enter credentials. If you get successfully authenticated you'll be redirected to Airflow 3 UI with a JWT token.

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[pierrejeambrun]

Thanks for the details. Those are answering the question I had on the related PR for the SimpleAuthManager route conversion to FastAPI.

Thanks.