Draft: Move BBM sub-batching into a wrapper

What does this MR do and why?

Related to #357033 (closed)

Continuation of the work from !83658 (closed). Add a new wrapper around job execution which performs the sub-batching and calls the job for each sub-batch.

In the future we'll have to expand on this concept. At a minimal I think we'll need a setup function to call on the job before starting the batching, and a way for the job to provide a relation to be merged into the sub-batching. Both of those should be easy to add in a future iteration.

It would also be helpful to add a cop so background migrations inherit from one of the existing base classes (preferably SubBatchedJob). Right now we have a few different combinations of how the jobs can be written, and we should standardize on the approach.

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