Ability to set shareProcessNamespace: true in containerSetTemplate Argo WFs (multi-container pods)

# Summary

I have been messing around with containerSetTemplates. It is a useful feature so far! I think something that would make this feature more flexible is allowing to share process namespace within the pod like you can a regular Kubernetes pod.

# Use Cases

For my situation:
I have a multi-container step in an argo workflow where I am doing some computational work. I need to be able to watch for certain processes to finish. This triggers a sidecar to push the results from the main container to my cloud storage provider. This is just one use case but I'm sure others may find it useful!

I have used the shareProcessNamespace field in Kubernetes Pods and Jobs a bunch.

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**Message from the maintainers**:

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Start by tracing how containerSetTemplates become multi-container Kubernetes pods, then compare that path with the shareProcessNamespace field used by regular Pods and Jobs. Done means a containerSetTemplate workflow can set shareProcessNamespace: true and the resulting pod shares its process namespace across containers, with coverage for the new behavior.

Written by the indexing model from the issue text.