serializeToCommandDoc() takes ExpCtx apart from AggRequest in order to serialize query settings into request body.

serializeToCommandObj() is currently not doing that. This was not critical for propagating query settings as part of relevant aggregation requests, as this method seems to be used only for change streams, which do not apply query settings.

This is not an issue per se, but an improvement. We need to either keep these methods consistent or keep only one or make one method use another.