Allow AI interactions to be proxied through Zed's server so you don't need an API key

[nathansobo]

Co-authored-by: Antonio [email protected]

Resurrected this from some assistant work I did in Spring of 2023.

• [x] Resurrect streaming responses
• [ ] Use streaming responses to enable AI via Zed's servers by default (but preserve API key option for now)

Release Notes:

• You can now use AI in Zed without an API key.