Replace setup.py with pyproject.toml (#9021)

* Create pyproject.toml
* Implement a custom build backend (see below) in packager directory. Build logic from setup.py has been refactored and migrated into the new backend.
* Tested: pip wheel . (build wheel), python -m build --sdist . (source distribution)

plugin/federated/README.md

@@ -19,7 +19,7 @@ cmake .. -GNinja \
  -DUSE_NCCL=ON
 ninja
 cd ../python-package
-pip install -e .  # or equivalently python setup.py develop
+pip install -e .
 ```
 If CMake fails to locate gRPC, you may need to pass `-DCMAKE_PREFIX_PATH=<grpc path>` to CMake.