Add ONNX flavor support

[aalkilani]

Currently ForestFlow only supports H2O flavors. This is extensible and we'd like to add support for ONNX-based models to cover a wider variety of frameworks that know how to export to ONNX.

This SHOULD look into runtime/platform considerations and provide switches for GPU support for example.

[EmergentOrder]

+1

You may find my project ONNX-Scala useful here.

[EmergentOrder]

Ahh, I see MLflow has ONNX support now, however it seems to be limited: https://github.com/mlflow/mlflow/issues/3229