Add model prop to useChat hook to remove the need for a backend

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Add model prop to useChat hook to remove the need for a backend

This PR introduces the model prop to the useChat hook, enabling direct interaction with Language Model V1 compliant AI providers (e.g., Google, Groq, and other models from @ai-sdk) directly from the client-side, thus removing the need for a dedicated backend in simple use cases.

Addresses: #5140

Changes:

• Added model prop to UseChatOptions: The UseChatOptions interface now accepts an optional model property of type LanguageModelV1.
• Conditional API Call: The triggerRequest function now checks if a model is provided. If so, it uses the model's doStream method to interact with the AI provider. Otherwise, it defaults to the existing callChatApi function, which requires a backend endpoint.
• Streaming Implementation: When a model is provided, the PR implements a streaming mechanism using TransformStream and WritableStream to handle the response from the AI provider. The response is processed chunk by chunk, updating the chat messages in real-time.
• Updated chatKey: The chatKey used for SWR is updated to include the model instance if it is provided, ensuring unique cache keys for different model configurations.
• Typescript improvements: Added the language model types to the existing types.

Usage Example:

"use client";
import { createGoogleGenerativeAI } from "@ai-sdk/google";
import { useChat } from "@ai-sdk/react";

// Existing function, I made no changes here
const google = createGoogleGenerativeAI({
  apiKey: "(pass the key from somewhere)",
});

function ChatComponent() {
  const { messages, append, input, setInput, handleSubmit } = useChat({
    model: google("gemini-2.0-flash-001"), // Pass the model instance - this is the change I made
  });

  return (
    <div>
      {/* Some UI */}
    </div>
  );
}

export default ChatComponent;

Super open to changing this if it's not what you had in mind. 😀

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Please do not merge this PR yet. I need to do some more tests. 🫤

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I'm done testing, ready for feedback 👂