https://jira.mongodb.org/browse/SERVER-109569 introduced the concept of Source stages to the API.

As part of this ticket:
1) Introduce MongoExtensionAggregationStageDescriptor::create_executable_stage() to the public API
2) Add a unit test which implements a Source stage in the C++ SDK, implement the executable stage required for the source stage, implement create_executor() and test the get_next() method.
4) Add support for create_executor in the Host adapter code.