KEP-2170: Create model exporter for checkpointing and training output

[andreyvelich]

As we discussed before, as part of the Training V2 APIs we want to design and implement model exporter sidecar which helps users to make checkpointing during distributed training and exporting the trained/fine-tuned model: https://github.com/kubeflow/training-operator/pull/2240#issuecomment-2321081416.

/area storage

[saileshd1402]

/assign