Initial functionality

[cbartz]

This PR adds the basic functionality to support the SAML group attributes using Flask Multipass within Indico.
The current SAML Identity Provider in Flask Multipass does not support the SAML group attributes, see this issue for further explanation.

Implementation notes:

• Groups are stored/cached in the Indico provided database, as Indico does not do this for external auth providers (see this issue), and group membership information would otherwise be lost on a server restart/crash.
• Group membership information is updated on every user login.
• Currently the package is designed as an Indico plugin and the code directly uses the SQLAlchemy instance from the Indico package, but this could be extended to be independent of Indico to be used for other services.
• The code has only been tested locally and not yet with a real SAML IdP service, so it should be considered experimental. I have added a warning to the README.

[github-actions]

license-eye has totally checked 36 files.

Valid	Invalid	Ignored	Fixed
24	1	11	0

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• flask_multipass_saml_groups/migrations/20230725_1640_ae387f5fc14a_initial_migration.py

[arturo-seijas]

LGTM!

[mthaddon]

We're missing docstrings for a few methods - are there plans to add those later or is that something that could be addressed now?

[cbartz]

We're missing docstrings for a few methods - are there plans to add those later or is that something that could be addressed now?

Any missing docstrings should be docstrings equivalent to the base class docstring. I avoided copy & paste there.
I always mention it on the first disable rule # noqa: D102,DCO010 docstring in base class, but did not repeat the reason "docstring in base class" everywhere.

[mthaddon]

Looks good to me, thanks. And thanks for the thorough PR description.