Update tutorial code in ros2 branch and host ros2 tutorials in this repository

[scpeters]

There are ROS 1 tutorials hosted on the ROS wiki at http://wiki.ros.org/image_transport/Tutorials, but I don't see any ros2 tutorials hosted anywhere. Also, the tutorial code exists on the ros2 branch, but it still includes ros.h, so it needs to be ported to ros2 as well.

[jacobperron]

I think it would be better if the tutorial code was actually built as part of this repository. I suggest making it a sibling of the image_transport package so that tools like colcon will pick it up and build it.