As the next part of implementing change streams, we need to define a ChangeStreamGetMore struct that implements Operation. This operation should wrap the GetMore operation, calling Aggregate::build and Aggregate::handle_response in its own respective build and handle_response methods.

Additionally, the poll_next methods on the ChangeStream's Stream implementation should be updated to create and execute a ChangeStreamGetMore operation, following the pattern used in the Cursor's poll_next implementation.