[REQ] Add the 'impl' directory to explain how to use python-fastapi

Is your feature request related to a problem? Please describe.

Thanks for the amazing project, I am a Python user, I want to use the python-fastapi generator to generate some Python code with the fastapi framework supported. But I failed to run it, the error log is something like "AttributeError: module 'openapi_server' has no attribute 'impl'". This might be a hint that my own implementation is needed to override the default (empty) implementations generated, However, I have tried many implementation strategies, and my own implementation still can not be successfully invoked. I am wondering if it is possible to provide a 'impl' directory with a basic implementation under samples/server/petstore/python-fastapi/src/openapi_server to demonstrate how it works.

Many thanks!

Start at samples/server/petstore/python-fastapi/src/openapi_server and inspect how the generated server package exposes implementation modules. Reproduce the reported AttributeError with the python-fastapi petstore sample, then add a basic impl directory that demonstrates overriding the default empty implementations and verify the sample invokes it successfully.