[FR] Additional Python feature plans

I think the following points might benefit GBM's Python distribution. They are all optional, but with varying degrees of usefulness:

- [x] Adding a modern build system-compliant (PEP517/518) `pyproject.toml`.
- [ ] Including all C++ source and header files into the wheel, along with the `cmake` directory; that way, Python people can include GBM as a build-time dependency and e.g. roll their own bindings. `nanobind` does this, specifically for integrating with `scikit-build`, for Bazel, it should work the same way.
- [ ] Adding a `benchmark.parametrize` decorator similar to `pytest.mark.parameterize` or `absl.testing.parameterized`, so that users don't have to `register` each benchmark by hand for each set of keyword arguments in the cartesian product.
- [ ] Making flag parsing with `absl` optional by opening up the flag parser as a configurable element of the benchmarking workflow.
- [ ] Consolidating all per-benchmark arguments in a single decorator to avoid excessive decorator stacking.
- [ ] Adding better IDE support by providing docstrings to the bound Python objects.

I will tackle them as I get the time, especially the registration QoL improvements might be worth it in my opinion.