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.

Start with docs/ldap.md and the ldap:AttributeAddUsersGroups section to understand the documented limitation. Then trace how IdP metadata and the selected authentication source are represented, since the issue identifies missing framework information but no implementation entry point or test. Done would require an agreed design, support for ldap:LdapMulti, and updated documentation.