fetch signal to abort and OpenAI stop signal

[jondwillis]

• [x] AbortController for fetches
• [ ] https://js.langchain.com/docs/modules/agents/additional_functionality#cancelling-requests

[jondwillis]

according to langchain docs (somewhere deep that I lost), some chains cannot be aborted.

Currently, on the server-side, there is no easy way to determine if the request has been aborted, even though we are sending the signal along from the request to the langchain calls