Create oplog fetcher for resharding

The oplog fetcher for resharding should take the aggregation pipeline from SERVER-49891, run it against a particular donor, and insert the returned documents into the oplog buffer collection on itself (a recipient).

• Serialize the aggregation pipeline from createOplogFetchingPipelineForResharding() using Pipeline::serializeToBson() and use the serialized form to create an AggregationRequest.
• Add the necessary read concern and hint to the AggregationRequest.
• Run the aggregate command on the remote donor. We likely don't want to use ClusterAggregate::runAggregate() because we want a recipient to target each donor with a unique aggregation pipeline (unique due to different resume points).
  • One option may be to use ShardRemote::runCommand() and Fetcher like ShardRemote::_runExhaustiveCursorCommand does.
  • Another option may be to use DBClientCursor like OplogFetcher does. This is perhaps the only way to take advantage of the QueryOption_Exhaust option.
• Use Collection::insertDocuments() to insert the documents into the new sharded collection.