[DRAFT] Restructuring code to include LightGBM as a dependency

[reluzita]

To run tests: pytest tests/test_cmake_build.py tests/test_constrained_objectives.py tests/test_num_constraints.py tests/test_proxy_loss_effect.py

To compile C code:

cd native
mkdir -p build && cd build
cmake ..
make -j4

[TaiPee]

Tested the build and ran the tests. A couple of notes on what was needed to get everything working:

Installing dependencies

The dev extras need to be installed explicitly since hypothesis (used by most of the test files) is
under [project.optional-dependencies]:

pip install -e ".[dev]"

Placing lib_fairgbm.so where Python can find it

Running cmake .. && make -j4 in native/build/ compiles successfully, but the .so ends up in
native/build/ rather than the fairgbm/ package directory where _lib.py looks for it. Using pip install
-e ".[dev]" handles both the build and placement in one step. Alternatively, a manual copy works too:

cp native/build/lib_fairgbm.so fairgbm/

With those two things in place, all 19 tests pass.