Sub-Templates that can be merged with templates during creation

# Summary

It would be awesome to have a sub-template configuration that could be included/merged with an existing templates. (Kinda like Kubernetes PodPresets)

# Use Cases

Examples:
* Node Selector / Toleration configuration to get a workflow step to run on specific nodes
* Sidecar configuration to include an SQL proxy sidecar

I was thinking about this from the standpoint of creating a mutating admission controller webhook but couldn't find a decent one that did what I wanted. It would be ideal if this could just be a resource where you could reference a partial template that gets merged.

The tolerations/node selector config could schedule for lots of different node types and with a simple annotation you could send a workflow to a specific node. (In my case, I want to keep the nodes shutdown unless I'm running the workflow)

Also, I would like to be able to easily inject proxy sidecars into workflows without having to specify the full configuration.

Another option would just be a way to include parametrically include yaml when writing templates.

If there's a way to do this already, I haven't found it yet.

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**Message from the maintainers**:
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Review the existing workflow template creation and Kubernetes integration, then compare the proposed sub-template resource, merge behavior, and parameterized YAML alternatives. The work is complete only when one approach is specified and implemented with a clear way to inject scheduling settings or sidecars into workflows without duplicating the full template.

Written by the indexing model from the issue text.