Make streaming vercel compatible

[WonderPG]

No description provided.

[jankrepl]

Thank you for doing this @WonderPG

Note that for the this to be compatible with the vercel UI SDK more work needs to be done

1. A special header needs to be inserted: https://github.com/vercel/ai/blob/main/examples/next-fastapi/api/index.py#L134C5-L134C55
2. Different request body https://github.com/vercel/ai/blob/main/examples/next-fastapi/api/index.py#L22-L23

I know I know, this is a lot of changes. I am fine with this entire vercel compatibility being just a side project and we don't have to merge to main if we don't like it. We can work on the above together.

[jankrepl]

We moved this to a different repo. Closing.