[RFC] PX4 / ROS message version compatibility

[bkueng]

With the introduction of using PX4 message definitions seamlessly on companions via ROS 2 DDS bridge, the question of message compatibility arises. Specifically, there is a need for the following use-cases:

• Allow a set of companion/linux applications compiled with different sets of ROS message versions to communicate with a specific PX4 version
• Allow a companion/linux application to communicate with different versions of PX4, without having to recompile the application or PX4

The details are outlined in this document, you're welcome to comment: https://docs.google.com/document/d/18_RxV1eEjt4haaa5QkFZAlIAJNv9w5HED2aUEiG7PVQ/edit?usp=sharing

As part of that I think it makes sense to split off a core message set and review the individual message content. These can then be versioned.