Create py.typed file in generated code directory

[mauruns]

Summary

mypy reflection on generated servicer classes

What is the feature request for?

The core library

The Problem

mypy cannot reflect on classes that are not recognized as properly typed. Providing the py.typed file enables mypy to search the Python files and reflect on the used types correctly. betterproto2 itself provides this file, the compiled output does not unfortunately.

The Ideal Solution

Generate an empty py.typed file on compilation

The Current Solution

Users need to create the file themselves after the fact

[AdrienVannson]

This is a good idea, but there is still a few typing errors that remain in the generated files. I think it's probably better to fix them first.