[WIP] Collision avoidance library addition example

[MaEtUgR]

@baumanta I took the time now to prepare this example for what I meant in my review here: #10785 (review)
I hope this helps you. If I didn't do any mistakes it should do the exact same thing as before but is a lot more modular.

Notes:

• I know that I only added the CollisionAvoidance library to be used in the FlightTaskManualPosition. It's because it's currently only used there. If it will be used for other tasks as well it's easy to adjust. Just move the object to the common ancestor which may as well be the base class.
• What I didn't change here is where the files are and the CMakeLists. I would suggest to handle it exactly like these libraries here: https://github.com/PX4/Firmware/tree/master/src/lib/FlightTasks/tasks/Utility
• Maybe there needs to be an unpublish on flight task destruction for the publications inside the library, I didn't look into that

FYI @mrivi

[MaEtUgR]

content of this branch got cherry-picked