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

As part of this ticket:
1) Introduce MongoExtensionAggregationStageLogicalStage::compile() 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 compile() and test the get_next() method.
3) Add support for compile() in the Host adapter code.