Fixes #60922

The Keycloak authentication provider generates HTTP redirect URLs even when
running behind an HTTPS reverse proxy. This occurs because the login route
uses request.url_for() to generate the callback URL, which doesn't respect
proxy headers like X-Forwarded-Proto by default.

This fix configures Airflow to respect proxy headers by adding support for:

1. Uvicorn's --proxy-headers flag with FORWARDED_ALLOW_IPS environment variable
2. Alternative ProxyFix middleware configuration

Both approaches enable the Keycloak provider to correctly generate HTTPS
redirect URLs when deployed behind nginx ingress or other reverse proxies.

Testing:

• Verified redirect_uri uses HTTPS when proxy headers are configured
• Confirmed backward compatibility with existing deployments