Support update with aggregation pipeline in modify method

[gowthamvbhat]

Currently updating with an aggregation pipeline is supported in update() and update_one() methods.

This PR add the same support in modify() method by following the original PR - https://github.com/MongoEngine/mongoengine/pull/2578

Related issue - https://github.com/MongoEngine/mongoengine/issues/2873

Please let me know if this requires any other changes.