Implement DocumentSourceExtension::distributedPlanLogic() to enable extension aggregation stages to participate in sharded execution planning by managing the MongoExtensionDistributedPlanLogic -> DocumentSource::DistributedPlanLogic conversion.

This will require overriding distributedPlanLogic() in DocumentSourceExtensionOptimizable to convert VariantDPLHandle elements into DocumentSource instances, then constructing the host's DistributedPlanLogic with shardsStage, mergingStages, and mergeSortPattern.