[Feature] Support for EventSource

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Description

The Worker platform already supports [EventSource ](https://developers.cloudflare.com/workers/runtime-apis/eventsource/) but it's not exposed in the Rust API.

Most LLM APIs provide a way to stream responses using SSE, so having support for this would make it simpler to manage those streams in Rust. Currently we need to implement our own stream handling to go from bytes to SSE events.