ipalib: introduce API schema plugins
    
    Add a set of plugins for API schema introspection. This includes:
    * a set of commands for command introspection
      (`command_find`, `command_show`, `command_defaults`),
    * a set of commands for command param introspection
      (`param_find`, `param_show`),
    * a set of commands for command output introspection
      (`output_find`, `output_show`),
    * a set of commands for help topic introspection
      (`topic_find`, `topic_show`),
    * a command to get the full API schema in one call (`schema`).
    
    https://fedorahosted.org/freeipa/ticket/4739
    
    Reviewed-By: David Kupka <dkupka@redhat.com>