Support PerpetualBooster

[deadsoul44]

Hello,

I am the author of PerpetualBooster:
https://github.com/perpetual-ml/perpetual

There is some demand from community to add onnx support for PerpetualBooster:
perpetual-ml/perpetual#1

I think the right place to add onnx support is here rather than PerpetualBooster repo.
I can try to implement it and create a pull request if you are willing to review it.
Tree structure of PerpetualBooster is very similar to XGBoost. I guess it shouldn't take too much effort.

[xadupre]

perpetual is outside the scope for this library. Can you leverage one of the examples which shows how to extend the list of converters: https://onnx.ai/sklearn-onnx/tutorial_2_new_converter.html.