Extract batched migration execution as dedicated worker

What does this MR do and why?

This MR is the first step towards Execute batched background migrations in parallel (#372316 - closed).

There is no change in the current behavior, we extract the execution of batched background migration in dedicated worker that is executed synchronously. In future change we'll convert this worker in limited capacity worker, so that we can execute multiple migrations in parallel. Here is the plan - #372316 (comment 1155327470).

Initially I started doing this all in !100240 (closed), but realized this will result in a very big MR that will be hard to review and ship, so decided to break the work down in smaller chunks.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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Related to #372316 (closed).