The changes in this commit make it compile with ROS2 Crystal Clemmys and
current Cyclone DDS. The RMW interface of ROS2 was modified in some
ways and extended in some other ways since Bouncy Bolson; and similarly,
Cyclone now has a somewhat reasonable interface for custom sample
representations and serialization, but the code in this commit probably
contains mistakes in using it.
Therefore, the expectation should be that this doesn't actually work
just yet, though it probably is quite close. As the old state wouldn't
build at all with any version of Cyclone DDS except the early commits,
this is significant progress already.
