Per our latest design discussions, this is the latest state of the MongoExecutableAggStageVTable to implement:

• MongoExtensionStatus* (*open)();
• MongoExtensionStatus* (*reopen)();
• MongoExtensionStatus* (*close)();
• MongoExtensionStatus* (*attach)();
• MongoExtensionStatus* (*detach)();

As part of this ticket, update MongoExecutableAggStageVTable to include the following methods (see TDD for more details):

• MongoExtensionStatus* (*dispose)();
• MongoExtensionStatus* (*open)();
• MongoExtensionStatus* (*reopen)();
• MongoExtensionStatus* (*close)();

Also do the following:

1) Implement the C++ SDK Boundary object (i.e adapter), as well as provide a base class for extensions to implement their own executable stage.

2) Implement the C++ Host Adapter.

3) Write unit tests for the C++ SDK Boundary object, as well as the host adapter.