How to use this library in cmake build-system

[KuiWei004]

as the header

[rwgk]

Sorry no cmake support here, similar to how it used to be for pybind11_protobuf until someone contributed pybind/pybind11_protobuf#73.

I'll try out and merge contributions here, too. I know only very little about cmake, which means I cannot help much with the details, but if we have something that runs with GHA I'll merge it.