Feature request

Feature description

Allow to pass an exact topic name to rcl and create a publisher/subscriber with that exact topic name at the DDS level. This will allow for better integration with native DDS systems
Currently users can bypass the custom ROS prefix added to the topic names, using the avoid_ros_namespace_conventions flag, but this doesnt allow to bypass name expansion into a fully qualified name.

Follow up of #476, particularly this discussion ros2/rmw_fastrtps#192 (comment)

Implementation considerations

RCL could pass both the name provided by the users as well as the fully qualified name to rmw. And the each rmw implementation would create a topic using on or the other based on the value of the avoid_ros_namespace_conventions provided in the qos profile