ability to transform response from backend in useChat

[narasimhajupally]

Feature Description

a flag to disable automatic message insertion into messages[] state in useChat when handleSubmit or append is used.

Use Case

if /api/chat endpoint is returning a custom response that includes the response from LLM, the frontend will be able to pick the message content manually and update the messages[] state in useChat hook using onResponse callback in UseChatOptions

Additional context

No response