Modify response in middleware

[Netail]

Goals

Being able to modify the response of a API or page in the middleware

Background

Currently, Nextjs' middleware only sits before page renders (only handling incoming requests), but there's no way in NextJS to modify the headers or body based on the app's response.

E.g. set a certain header in-case the app's page returns a certain HTTP response code. Or change the HTTP code to something else.

Proposal

Backwards compatible

1. Different middleware file for responses
2. Being able to export another middleware function for responses in the original middleware.ts

Breaking

1. NextResponse.next() actually returns the app's response