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>