chore: improve schema validation

[dpgaspar]

Description

Improves schema validation and adds optional flag for allowing alternative providers

ADDITIONAL INFORMATION

• Has associated issue:
• Is CRUD MVC related.
• Is Auth, RBAC security related.
• Changes the security db schema.
• Introduces new feature
• Removes existing feature

[harshach]

hi @dpgaspar this schema validation is causing an issue in our applications

 File "python3.9/site-packages/flask_appbuilder/security/schemas.py", line 24, in validate_provider
    if not current_app.appbuilder.sm.api_login_allow_multiple_providers:
AttributeError: 'AirflowSecurityManager' object has no attribute 'api_login_allow_multiple_providers'

can this be gated with

if hasattr(current_app.appbuilder.sm, 'api_login_allow_multiple_providers') and not current_app.appbuilder.sm.api_login_allow_multiple_providers:

or do you see any better fixes on the application side. Thanks

[jedcunningham]

Hey @harshach, Airflow maintainer here. I'm curious what use cases you have that require you to hit that /login endpoint? I'm considering disabling it by default.