[PoC] Flask Session 0.6

[Taragolis]

related: #36897

Just an attempt to make work with Flask-Session 0.6.0 and mostly it is unsuccessful.
Random error occurs during the login mechanism, see: tmux-webserver-buffer.txt

Some interesting findings is [database] sql_alchemy_pool_enabled set to False (Postgres only, MySQL will fail with error due to outdated Flask-SQLalchemy) then everything seems works fine, as soon as I return to the default value then it fail with random error, seems like Flask-Session 0.6.0 miss some commit to the DB, and better to wait until 0.7.0

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[Taragolis]

This PR was just to show that something wrong with Flask Session 0.6.0 and we should not to spend much time on it and better wait next version.

[potiuk]

Yes. 0.6.0 is completely broken.

[Lxstr]

I have 0.7.0rc1 now available. I would appreciate any help with the SQLAlchemy interface. I do not personally use SQLAlchemy but I've tried to fix a number of issues and also add cleanup functionality.

It should only save sessions when there is an actual value set to the session. So there should be no more need for Airflow's SessionExemptMixin.

Also note I have had to change to interfaces from flask_session.SQLAlchemySessionInterface to flask_session.sqlalchemy.SqlAlchemySessionInterface for maintainability. The params should be backwards compatible though.

[Lxstr]

Unreleased docs at https://flask-session--212.org.readthedocs.build/en/212/