Support anonymous responses from the server

[monolithed]

Goals

It would be nice to have anonymous responses from the server. This is crucial for application security. I suggest providing the ability to change the names of the following variables:

NEXT_ROUTER_STATE_TREE
NEXT_ROUTER_PREFETCH_HEADER
NEXT_URL
NEXT_LOCALE
_next/static

Non-Goals

No response

Background

As an alternative, reverse-proxy is currently being used, but it's an uncontrolled approach that requires constant support in the form of configuration writing and testing.

#5602

Proposal

This could be configured either through individual variables in the config, such as 'poweredByHeader', or through a general flag with arbitrary values.