1. I'm not using git-subrepo for this because most of the protobuf-c
git repository is for the protocol buffers compiler integration,
which we do not need because nats.c already has the generated files
in its source tree.
2. The protobuf-c runtime library is pretty simple, so maintaining its
build process should be quite straightforward. It seems stable as
well, which should simplify the maintenance burden.
3. Not having streaming support is the last major nats.c feature
missing from this project (as far as I am aware). Building this
functionality into the library, even if a nice zig API is missing
(as it is with jetStream as well), can still allow a determined
user to wrap and use the functionality while benefitting from the
zig build system and package manager, which is a win.