[REQ] [python-fastapi] Automatically importing sub-packages modules in implementation package

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

Currently only modules placed in the package {{fastapiImplementationPackage}} are automatically imported: https://github.com/OpenAPITools/openapi-generator/blob/ba169342a1154d8f6694dbb9e759b03aa56e1619/modules/openapi-generator/src/main/resources/python-fastapi/api.mustache#L33-L35

A small improvement would be to also import automatically modules in sub-packages of FastAPI implementation package.

Describe the solution you'd like

This could be supported by replacing pkgutil.iter_modules with pkgutil.walk_packages in modules/openapi-generator/src/main/resources/python-fastapi/api.mustache.

Before:

ns_pkg = {{fastapiImplementationPackage}}
for _, name, _ in pkgutil.iter_modules(ns_pkg.__path__, ns_pkg.__name__ + "."):
    importlib.import_module(name)

After:

ns_pkg = {{fastapiImplementationPackage}}
for _, name, _ in pkgutil.walk_packages(ns_pkg.__path__, ns_pkg.__name__ + "."):
    importlib.import_module(name)

Start in modules/openapi-generator/src/main/resources/python-fastapi/api.mustache and inspect the existing automatic import loop around pkgutil.iter_modules. Update the template so nested modules in the FastAPI implementation package are included, then verify generated output imports modules from sub-packages as well as the top-level package.