Add feature flag option to worker idempotency

What does this MR do?

When trying to make PipelineProcessWorker idempotent (!62410 (merged)), I thought that it might be better if we could control this with a feature flag. I made the same changes over there but it is better to have it in a separate MR.

This MR adds the support of feature flag usage to the idempotent! method of workers.

Example

class PipelineProcessWorker
  include ApplicationWorker

  deduplicate :until_executing
  idempotent! feature_flag: :ci_idempotent_pipeline_process_worker

Does this MR meet the acceptance criteria?

Conformity

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• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
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Security

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