Add base class for BBM to abstract sub-batching

What does this MR do and why?

Related to #357033 (closed)

Taking a different approach based on !83658 (comment 902874547), this MR adds a base class that batched migration jobs can inherit from.

In the default case, they can use the provided implementation of perform and only provide an implementation of perform_sub_batch. But, in the complex case, they can override perform as well to implement whatever logic is needed.

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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