AD: Add Global Catalog usability check in subdomain code by looking at the schema
    
    Addsa a new tevent request which checks for the presence of uidNumber
    and gidNumber under the schema naming context, which is typically
    cn=schema,cn=configuration,$BASEDN. For both objects representing each of
    the attributes, the isMemberOfPartialAttributeSet attribute is requested. If
    this attribute is set to TRUE, then the attribute corresponding to this
    schema object had been replicated to the Global Catalog.
    
    Because the isMemberOfPartialAttributeSet is not replicated to the GC
    itself, we use the LDAP connection for the search.
    
    Related:
    https://pagure.io/SSSD/sssd/issue/3755
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>