ChangeStreamReaderBuilder exposes getControlEventTypesOnDataShard() and  getControlEventTypesOnConfigServer() as part of its interface. These methods returns a set of strings, which corresponds to the control event types.

As part of ShardTargeter implementation we will rely on ControlEvents in order to handle them appropriately as part of handleEvent() method.

This ticket will introduce the ControlEvent variant with a set of ControlEvent structs, which will expose opType, such that one could use it in the getControlEventTypes() methods as well as factory method that would allow to create a ControlEvent out of Document