Somehow initialize ping_message function to avoid magic numbers.

[patrickelectric]

https://github.com/bluerobotics/ping-viewer/pull/522/files#r297255444

[jaxxzer]

This is the intended use case of this constructor. I can define on my own a ping message and populate it/access it by hand. The type of application is less common, as in this never-documented legacy use-case, or if I just wanted to experiment with firmware without modifying three repositories in the build system.