docs: list of HTTP headers used by next

[vasimi]

Goals

1. document all headers used by Next to make it easy for people who manage next behind a reverse proxy to pass the correct headers

Non-Goals

No response

Background

I was debugging server actions that didn't work in my Next app running in AWS behind CloudFront (CDN) until I finally discovered that it's because CloudFront strips all the headers except for those specified in caching policy. I tried looking up an exhaustive list of headers used by Next, but couldn't find one, so I had to settle with copying the headers actually sent with the Action request.

There's unfortunately no guarantee that these headers won't change in the future and I have no other way of finding out than deploying the app and see if it breaks. If they were documented, I could just check the docs and update my caching policy (or any other reverse proxy config).

Proposal

A new section in the docs with an exhaustive list of all the headers Next is using (not only the next-* ones, but also the ones that are, for example, used for determining whether the request can be served from cache). If making a list of all the headers would be too hard (or it'd be too hard to maintain), at least a list of headers that can break next if they're not forwarded would be nice.