Make change stream control events distinguishable by their type.
The type should be a new metadata field exclusively set on change stream control events.
For the project, add the new metadata field and give every existing control event the type topology.
Once we add the control event for endOfTransaction accounting events, they will get the type accounting.

This work only introduces the new type metadata field for control events and unit-tests them. The control event type will be read and used by follow-up tickets in this project.

More details can be found in the technical design here.