Automatic .pyi stub file generation

[cmpute]

This is a pretty large feature if it's going to be supported, but I still think it's very useful for users. Usually, I have to write the stub file manually and update the definition carefully against the implementation when changes are added. It will be really helpful if the stub files can be generated automatically. My rough idea is that this should be feasible through parsing the rustdoc json output.

[cmpute]

I'm using maturin build PyO3 libraries, so I post the feature request here. Not sure if it's the proper place to request, maybe this feature should be handled by pyo3 side?

[cmpute]

I found this issue PyO3/pyo3#2454, sorry for the noise!