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/conda_env/sdist_test.yml

@@ -8,5 +8,6 @@ dependencies:
 - wheel
 - cmake
 - ninja
+- python-build
 - c-compiler
 - cxx-compiler