Add stream handler to Agent executor

As I delve into the development of the Rust port for the Lagchain library, I've identified a crucial feature that appears to be absent in the original Python implementation—native support for streaming the final responses of agents. This functionality is particularly important for applications requiring real-time data processing and analysis. Inspired by a tutorial on streaming Lagchain agent outputs (video link below), I believe incorporating this feature into the Rust port could significantly enhance its utility and performance.

Video Reference: [How to Stream Lagchain Agent Output](https://www.google.com/search?q=How+to+stream+lagchain+agent+output&oq=How+to+stream+lagchain+agent+output&gs_lcrp=EgZjaHJvbWUyBggAEEUYOTIGCAEQLhhA0gEJMTE1NTdqMGoxqAIAsAIA&sourceid=chrome&ie=UTF-8#fpstate=ive&ip=1&vld=cid:c9b5b8ca,vid:gr5CGL4_jpY,st:0)

Feature Description:
I propose we integrate a feature allowing for the asynchronous streaming of an agent's final response. This could potentially be implemented using Rust's robust asynchronous and concurrency features, such as futures and streams, to provide a high-performance, real-time data streaming capability.