feat: Allow multiple values for SSO attribute_requirements via comma separation (#17949)

In the current `attribute_requirements` implementation it is only
possible to allow exact matching attribute values. Multiple allowed
values for one attribute are not possible as described in #13238.

### Pull Request Checklist

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[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters)

)

---------

Co-authored-by: Sebastian Neuser <pzkz@infra.run>
Co-authored-by: Quentin Gliech <quenting@element.io>