[Feature]: PodSet Container to support Image and Command

What you would like to be added?

Problem: If we want to manipulate a Container or InitContainer on the JobSet, e.g., here we need to expose the Image and Command for the InitContainer, which are generally needed. We cannot use PodSet for InitContainer without this exposure. The ultimate goal is to have the PodSet interface map onto the actual Jobset (e.g., PodSet -> runtime.Container -> JobSet) with changed Image and Command. Note that tests can then be refactored too (reference)

Why is this needed?

Customization of Image/Command for InitContainers is generally desired. For the Flux Policy PR/integration we are not able to use PodSet without it: https://github.com/kubeflow/trainer/pull/3064#issuecomment-3815049315

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