spike: introduce WorkerStage abstraction

[yaauie]

This is a spike PR for underlying worker/pipeline changes to make pipelines recoverable in-place per #18534

[github-actions]

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[mergify]

This pull request does not have a backport label. Could you fix it @yaauie? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• If no backport is necessary, please add the backport-skip label

[elasticmachine]

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• Commit: 591d81c