chore: migrate DisaggregatedSet helper assets (#875)

* chore: migrate DisaggregatedSet helper assets

Signed-off-by: Kay Yan <kay.yan@daocloud.io>

* chore: add Apache license header to plan-steps

Add the standard boilerplate header to hack/plan-steps/main.go so it
matches the rest of the repository's Go sources.

Signed-off-by: Kay Yan <kay.yan@daocloud.io>

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Signed-off-by: Kay Yan <kay.yan@daocloud.io>