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)

tests/ci_build/test_python.sh

@@ -28,7 +28,7 @@ function install_xgboost {
   then
     pushd .
     cd python-package
-    python setup.py install --user
+    pip install --user -v .
     popd
   fi
 }