PVC Pool: cheap shared volumes

# Summary

PVCs are expensive and there maybe limits on providers to the total number create.

# Use Cases

Allow users to define a pool of PVCs which are allocated to workflows.

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**Message from the maintainers**:

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Start by tracing how Argo Workflows currently create and attach PVCs; the issue names no files, tests, or entry points. Clarify the pool lifecycle, allocation and release rules, provider limits, and workflow-facing configuration before implementation. Done means users can define a PVC pool and workflows can safely receive shared volumes.

Written by the indexing model from the issue text.