Once subpipelines with extensions are supported, update the constraints for the extension $limit stage appropriately (ex: LookupRequirement::kAllowed, UnionRequirement::kAllowed, etc.).

Update the relevant testing to measure perf of extension $limit against the existing $limit (ex: add a test case comparing perf of extension $limit in $unionWith against server $limit in $unionWith).

Other considerations: when will support for facet and transactions with extension stages be implemented?