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.

从 docs/ldap.md 和 ldap:AttributeAddUsersGroups 部分开始，以了解已记录的限制。然后追踪 IdP 元数据和所选身份验证源是如何表示的，因为该 issue 指出了缺少框架信息，但没有提供实现入口或测试。完成这项工作需要达成一致的设计、支持 ldap:LdapMulti，并更新文档。