Adds some missing aggregation stages

[aturki]

Q	A
Type	feature
BC Break	no
Fixed issues	#2063

Summary

This PR implements some missing aggregation pipeline stages.

Known missing stages are:

• $changeStream

[alcaeus]

The $changeStream aggregation pipeline stage was omitted on purpose, as the drivers expose this functionality through the watch methods on the Client, Database, and Collection objects. Is there a particular use case for adding this method to the builder?

[alcaeus]

Closing here. The $changeStream aggregation pipeline stage is meant to be used through the driver's watch method and not called individually.