Feat: Extend custom response handling and passthrough request headers

[onmotion]

Fix custom response handling and passthrough request headers

Motivation:

I like this library, but I found that the behavior of middleware differs from that of NextJS.

This PR addresses two main issues:

1. Request Headers Propagation: Previously, request headers were not being fully passed through or amendable within the middleware chain. For example, comparing with the behavior of nextjs proxy - if you return undefined or NextResponse.next() there, then by default all request headers will be passed to the route. In the nimpl-proxy-chain, they are null.
2. Custom Response Handling: When a middleware returns a custom response (e.g., a base Response with status 401 for a basic auth), the chain needs to recognize this as a final state to prevent unintended processing or overwriting by subsequent logic. Compared to Next.js behavior, if return a basic Response, it will be returned immediately "as is".

Changes:

• Passthrough Request Headers:

  • Updated collectData to initialize requestHeaders with the original request headers (req.headers) instead of an empty object.
  • Updated formatResponse to include request.headers in the NextResponse configuration, ensuring modified headers are forwarded to Next.js.

• Custom Response Logic:

  • Added logic to mark custom responses (where a body is present) with FINAL_SYMBOL in collectData.

• Examples:

  • Add the basic-auth example to demonstrate how to handle a custom Response within the chain.