Implement get_actual_qos() for subscriptions

[mm318]

Currently, publishers have the get_actual_qos() feature/function. It would make sense for subscriptions to have them, too.

[ivanpauno]

Similar comment about avoiding duplicated code that here.

[ivanpauno]

Why is this check needed? I think the one in L382 is enough. I know that the same was done in:

rmw_connext/rmw_connext_cpp/src/rmw_publisher.cpp

Lines 377 to 393 in 8e9e088

	DDS::DataWriter * data_writer = info->topic_writer_;
	if (!data_writer) {
	RMW_SET_ERROR_MSG("publisher internal data writer is invalid");
	return RMW_RET_ERROR;
	}
	DDS::DataWriterQos dds_qos;
	DDS::ReturnCode_t status = data_writer->get_qos(dds_qos);
	if (DDS::RETCODE_OK != status) {
	RMW_SET_ERROR_MSG("publisher can't get data writer qos policies");
	return RMW_RET_ERROR;
	}
	if (!data_writer) {
	RMW_SET_ERROR_MSG("publisher internal dds publisher is invalid");
	return RMW_RET_ERROR;
	}

But I don't think it has sense. If you agree, delete the duplicated check in the publisher function too

[mm318]

Done.

[mm318]

Refactored to avoid duplicate code. Thanks!

[ivanpauno]

CI: ros2/rmw#177 (comment)