fix: correctly set `x-forwarded-*` in Middleware (#57815)
### What?
Follow-up of #56797
While working on this, I noticed that some logic around stripping internal headers was duplicated, so I did some cleanup too.
### Why?
In #56797 we set these headers, but it only affected Route Handlers, Middleware is still missing them, which is a regression introduced in #52492
(Related: https://github.com/vercel/next-learn/issues/252)
### How?
Move to set these headers up to `base-server.ts` so they are present in Middleware too.
> Note: All headers are set with `??=` to respect the original value if set (with other words, only add these headers if they aren't set yet)
Closes NEXT-
Fixes #52266
