Add :none strategy for deduplicating Sidekiq jobs

What does this MR do?

Related to gitlab-com/gl-infra/scalability#195 (closed) as suggested in !39464 (comment 409139398)

Allows a job not to be deduplicated even though it is idempotent:

  class DummyWorker
    include ApplicationWorker

    deduplicate :none
    idempotent!

    # ...
  end

The default behavior for idempotent workers is to remove duplicated jobs, but in !39464 (merged) we have a different use case, we can have duplicated jobs which do unrelated work. So we need a strategy to cancel deduplication.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
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