Returns request types supported by a service desk instance.
        :param service_desk: The service desk instance.
        :type service_desk: ServiceDesk
        :rtype: List[RequestType]

def request_types(self, service_desk):
        """ Returns request types supported by a service desk instance.
        :param service_desk: The service desk instance.
        :type service_desk: ServiceDesk
        :rtype: List[RequestType]
        """
        if hasattr(service_desk, 'id'):
            service_desk = service_desk.id
        url = (self._options['server'] +
               '/rest/servicedeskapi/servicedesk/%s/requesttype'
               % service_desk)
        headers = {'X-ExperimentalApi': 'opt-in'}
        r_json = json_loads(self._session.get(url, headers=headers))
        request_types = [
            RequestType(self._options, self._session, raw_type_json)
            for raw_type_json in r_json['values']]
        return request_types