ROS2: Add UDP support

[kmhallen]

No description provided.

[clalancette]

This looks generally good to me, though I have a comment/question about the doWrite/doRead templated methods.

Can we use the asio::write() and stream_->async_read_some() ASIO methods for UDP? If not, why not? Isn't the point of ASIO to paper over these differences so we don't have to have separate template methods like this?

[kmhallen]

Some lower level functions weren't defined for UDP sockets with write() and async_read_some(). Comment out the added lines to async_worker.h to see the build errors. I couldn't find a better way.

[clalancette]

Some lower level functions weren't defined for UDP sockets with write() and async_read_some(). Comment out the added lines to async_worker.h to see the build errors. I couldn't find a better way.

All right, that's fine. I'm not that upset with having the extra template methods, it just seems odd given the usage ASIO. This looks good to me, thanks for the contribution!