https://jira.mongodb.org/browse/SERVER-109571 introduced the concept of Transform stages to the API.

As part of this ticket:

0) Implement HostExecutableAggregationStage in the Host adapter. Please refer to the design document for more details. 

1) Modify MongoExtensionAggregationStageDescriptor::create_executor() in the public API so it can accept a source executable stage.
2) Add a unit test which implements a Transform stage in the C++ SDK, implement the executable stage required for the transform stage, implement create_executor() and test the get_next() method.