Add support for protobuf to the codegen module

[dcbaker]

protobuf offers a frustrating experience with Meson because it wants to generate a mirrored directory structure that Meson doesn't like. We can wrap this up in a codgen helper to make everything much easier.

As an implementation detail, we may want to have the output be placed in the private directory with a method to it as an include. Unsure.

I don't use protobuf personally, though I can likely type up the code. I would need some test cases to help drive that.