Example or command line tool to show how to build `*_ddf.h` from `.proto` files

It would be great to have an example or a command line tool (`.sh`) to demonstrate how to generate `*_ddf.h` files from `.proto` files. It's necessary thing to know what properties do we have in C++/Java.

I didn't find examples in the Spine and Rive extensions either, so I conclude that the team is using some of their own tools to generate. It would be cool if you could share an example.

In the meantime, I have not been able to combine protoc and ddf_cxx for it to produce a proper result.

Many thanks!