GOWTHAM V BHAT

Currently [updating with an aggregation pipeline](https://docs.mongoengine.org/guide/querying.html#update-with-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...

The [MongoEngine documentation](https://docs.mongoengine.org/guide/querying.html#update-with-aggregation-pipeline) states that updating with an aggregation pipeline is supported in the `update()` method. Currently, this functionality is available in both `update()` and `update_one()` methods. However, the `modify()`...