Put the contribution reassignment autovacuum table health check behind a feature flag
What does this MR do and why?
This is a conservative measure to allow us to enable the flag if it's
determined that we should be delaying reassignments when tables are
being autovacuumed. We don't intend to release this flag and intend to
remove it a couple of milestones after the related
reassignment_throttling is released.
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Related to #525566 (closed)