mark_indices_as_ready task should use the event pattern to process

What does this MR do and why?

• Since we have a design pattern in the scheduling service to emit events I have extracted the logic in the mark_indices_as_ready task to an event worker. Secondly, the task mark_indices_as_ready now processes just one batch of indices on a single run. This will allow more controlled updating of indices and database transactions.

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Related to #510987