The goal of this ticket is to create a function which allows the aggregation pipeline to easily be sent to a remote (donor) shard and for the aggregation pipeline to be unit-testable with DocumentSourceMock. It is preferable to use DocumentSourceXX::create() functions (or DocumentSourceXX::parseFromBSON() when the former isn't available or is too tedious) rather than building it up with string concatenation. DocumentSources can be conditionally added to the Pipeline::SourceContainer, for example, to reflect a stage being added only when resuming on a new cursor.

std::unique_ptr<Pipeline, PipelineDeleter> createOplogFetchingPipelineForResharding(
    ReshardingDonorOplogId startAfter,  /* isNull() when not resuming */
    ShardId recipientShard
);

Some of these parameters are probably more appropriate to take by const-ref because their contents can only be copied into the Pipeline anyway.

Testing should cover the following types of oplog entries:

• Ordinary and retryable insert, update, or delete.
  • Includes formats for $set/$unset updates, replacement updates, and $v:2 delta updates (compressed pipeline updates).
• The preImageOpTime and postImageOpTime documents from retryable findAndModify operations.
• The final oplog entry marker from when the donor shard entered the "all new incoming writes as distributed transactions" state.
• Drop collection.
• All of the above for the matching (namespace, recipient), a different recipient, and a different namespace.