schema: add object class schema
    
    Support object classes defined by object plugins in API schema.
    
    Added new commands `class-show` and `class-find` to retrieve information
    about object classes. `param-show` and `param-find` now support both
    commands and classes.
    
    https://fedorahosted.org/freeipa/ticket/4739
    
    Reviewed-By: David Kupka <dkupka@redhat.com>