Add NextJsMiddleware

[mjnaderi]

This PR introduces an ASGI middleware that improves the Django-Next.js integration experience.

Key Improvements:

1. Simplified Setup Process

   • Replaces the manual configuration of NextJSProxyHttpConsumer and NextJSProxyWebsocketConsumer with a single middleware: NextJsMiddleware.
   • Provides a clean, declarative approach for integrating Next.js with ASGI applications.

2. Optimized Resource Management

   • Manages an aiohttp ClientSession throughout the application lifecycle using the ASGI lifespan protocol. The session is created during application startup and properly closed during shutdown, ensuring efficient reuse of HTTP connections when communicating with the Next.js server.

3. Less Dependencies

   • Implements NextJsHttpProxy and NextJsWebSocketProxy as pure ASGI handlers instead of Django Channels consumers.
   • Deprecates old proxy handlers.
   • Removes channels from dependencies.