aggregate method
Implementation
Future<List<MongoDocument>> aggregate(List<PipelineStage> pipeline) async {
List<dynamic> resultJson = await FlutterMongoRealm.aggregate(
collectionName: this.collectionName,
databaseName: this.databaseName,
pipeline: pipeline.map((doc) => jsonEncode(doc.values)).toList(),
);
var result = resultJson.map((string) {
return MongoDocument.parse(string);
}).toList();
return result;
}