New Feature: AggregatorChannel Add custom mode

name: add a custom mode for Agregrator mode
about: when using a task and reviewer workflow, the flat mode allow to get all history in the parent chat , and the nested mode only the latest message. so in the the case

• flat mode : I need to use the reducer to not take into account the reviewer comment in the parent chat
• nested mode : I only have the the reviewer comment, and I prefer the task result when the agent is completed

so my request is to have a custom mode , in order to select the message I want to display in the parent chat.

I would like to custom mode and and having a callback function in order to select the message visible in the parent chat . or to have the possibility to use chat reducer into the aggregator agent order to manage what agents will me part of the parent chat. but i prefer the custom selection mode.

Start by reviewing the AggregatorChannel implementation and its existing flat and nested modes. Clarify the callback or custom-selection API, how selected messages appear in the parent chat, and the expected behavior for task and reviewer workflows before defining completion criteria.