Add support for onnx export

[djdameln]

Users might want to be able to export their models in onnx format. While we do export the model in .onnx format when OpenVINO export is enabled, there is currenty no way to export just the .onnx file, without the Open VINO IR files. To support this, we could add the onnx key under optimization section in config.yaml, e.g.

optimization:
    onnx: true

To make it a bit more accurate, we could rename optimization to export