ROS2 support

[hortovanyi]

Are there plans to migrate this to ROS2?

[tonybaltovski]

Currently I don't have bandwidth but PRs are most welcome!

[mikeferguson]

As a note, I've done an initial port of um7 (and underlying serial library) here:

• https://github.com/mikeferguson/um7/tree/ros2
• https://github.com/mikeferguson/serial/tree/ros2