Expose a `AssistantStream` function

### Feature Description

I feel like it would make more sense to have a `AssistantStream` function that returns a Readable stream that you can chain with a `StreamingTextResponse` function.

We can still have that convenient `AssistantResponse` but I'd really need to import the Readable stream object in my case 🙏

### Use Case

_No response_

### Additional context

_No response_