Class DocumentSourceChangeStreamTransform parses the full change stream config in its createFromBson method.

It also informs the ExpressionContext about the change stream in that method.

In its constructor, the class also sets the initial PBRT in the ExpressionContext.

In its serialize method it also serializes the entire change stream configuration, and not just the configuration of the transform stage.

This should be improved so that the transform stage can be fed a minimal transform stage-specific config only.
We should also check all other change stream stages if they have similar issues.