Allow specifying custom image for each @step in the pipeline.

[Techn0logic]

python: 3.8.8
pip: 21.3.1
kubeflow-kale: 0.7.0

Currently it seems it's not supported to specify custom image for the step using @step decorator.

Desired functionality would be to specify image and other pod options for each step.

@step('my-sum', CONTAINER_IMAGE='my-custom-image-sum:latest', VOLUME_MOUNTS=['my-custom-volume1','my-custom-volume2'])
def task(a,b):
    return a+b