LightGBM: Support for outputting decision path and leaf

[ajryan]

Has any work already begun for outputting decision path and leaf data? I believe it would be similar to this implementation for decision tree and random forest in sklearn_onnx https://github.com/onnx/sklearn-onnx/pull/563.

One challenge appears to be onnxmltools does not have a story for converters that accept options. I see that sklearn implemented their own topology and Scope that exposes an options API.