Pause batched background migration when WAL pending to archive is above threshold

What does this MR do and why?

This builds on top of Pause data migrations upon active autovacuum (!85196 - merged), updates the code so that we can evaluate more than one indicator, and adds new indicator that checks number of WAL segments pending archival.

Related to #357249 (closed).

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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