Configure Flask app settings via prefixed environment variables in FAB provider

[aeroyorch]

During the review of #58293, a discussion came up around how the Airflow webserver Flask application is configured. At the moment, Flask-specific settings/behaviour are mainly provided through webserver_config.py.

This discussion is intended as a follow-up to explore whether Airflow should optionally support a second way of configuring some Flask app settings, using prefixed environment variables via app.config.from_prefixed_env() (Flask-Limiter also honors Flask’s configuration values.).

Motivation

Using app.config.from_prefixed_env() would allow selected Flask configuration options to be provided via environment variables, for example:

AIRFLOW__FLASK__RATELIMITER_STORAGE_URI=${AIRFLOW__CELERY__BROKER_URL}

This approach can be useful not only in the Airflow Helm chart, but more generally in declarative deployments where configuration is defined via environment variables and overridden per environment without changing Python files. It can also help avoid introducing imperative logic or environment-specific conditionals in code, which can make it harder to understand which configuration applies in a given setup. In addition, sensitive parts of the configuration can be managed separately using secrets.

At the same time, this is a proposal, and some users may not see additional value if their current setup already meets their needs.

Proposal

Allow the webserver to optionally load some Flask configuration values from environment variables using app.config.from_prefixed_env(), with a clear prefix such as AIRFLOW__FAB__, or even a more specific namespace (e.g. AIRFLOW__FAB__FLASK_SETTINGS, AIRFLOW__FAB__FLASK_CONFIG, AIRFLOW__FAB__CONFIG).

This mechanism would not replace webserver_config.py, which would continue to be the primary configuration mechanism. When both approaches are used, webserver_config.py would have precedence over environment-based configuration.

POC PR: #60073

[potiuk]

As dicussed in the original discussion - draft PR with POC would be nice to make the discussion concrete.