0) Introduce new stage type in public API header (kDesugar)
1) Introduce MongoExtensionAggregationStageDescriptor::expand() to the public API.
2) Add a unit test which implements a Desugar stage in the C++ SDK, and test the expand() method.
3) Add support for expand() in the Host adapter code.

Note, this ticket does not include adding support for registering a desugar stage in the Host. It should only add the new stage type in the C++ SDK, and add support for the new method in the Host adapter.

Don't worry about recursive desugaring as part of this ticket.