Replace boolean with enum argument in AttachWorker

What does this MR do and why?

Changes the unused AttachWorker argument from the boolean _replace_existing to a _params object that defaults to an empty object, so it can carry the attach mode values in the future, without changing the worker signature again. The argument is still ignored and nothing sends it yet, so there is no behavior change. Sending it starts in 19.6 to keep Sidekiq arguments compatible across updates.

Changelog: changed
EE: true

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