Have ldap:AttributeAddUsersGroups support ldap:LdapMulti

Hi, I noted that in the docs for ldap:AttributeAddUsersGroups, it is clearly stated:

Note: ldap:LdapMulti is not supported as the SimpleSAMLphp framework does not pass any information about which LDAP source the user selected.

And I understand that if you do have multiple LDAP instances, you can place your ldap:AttributeAddUsersGroups configuration in the service provider metadata and in that way control which LDAP instance is queried for groups when a user attempts to access a particular service.

But I do wonder if there is any appetite for adding support for ldap:LdapMulti? So that with ldap:AttributeAddUsersGroups configuration in the IdP metadata, groups will be queried from the authentication source (organisation) the user selected?

I'd be very happy to work on this if anyone feels it would be a useful feature to add.

Comienza con docs/ldap.md y la sección ldap:AttributeAddUsersGroups para entender la limitación documentada. Después, rastrea cómo se representan los metadatos del IdP y la fuente de autenticación seleccionada, ya que el issue identifica que falta información del framework, pero no especifica un punto de entrada para la implementación ni una prueba. Para darlo por terminado sería necesario un diseño acordado, compatibilidad con ldap:LdapMulti y documentación actualizada.