add --config-setting stub_suffix optional feature to create more distinct file names

[ppham-nv]

It would be useful to allow adding a stub_suffix to better distinguish between the stub tar ball vs real source tar ball. Example usage:

PIP_FIND_LINKS=`pwd`/../dist PIP_NO_INDEX=1 python -m build --sdist --config-setting source_wheel=torch-2.9.0a0+145a3a7bda.nv25.10-36781467-cp312-cp312-linux_x86_64.whl --config-setting stub_suffix=stub
* Creating isolated environment: venv+pip...
* Installing packages in isolated environment:
  - wheel-stub
* Getting build dependencies for sdist...
* Building sdist...
Successfully built torch-2.9.0a0+145a3a7bda.nv25.10.stub.tar.gz

[ppham-nv]

this is a file name change only pypi version still shows the version based on PKG-INFO so might need to rethink this. Will close this for now.