configurable request header for determining peer

[rnewson]

Overview

Allow peer determination by a custom header, useful when CouchDB is behind a reverse proxy or load balancer.

In the absence of the header the usual logic is used to determine peer (the x-forwarded-for header logic in mochiweb).

Testing recommendations

covered by tests.

Related Issues or Pull Requests

#6052

Checklist

• This is my own work, I did not use AI, LLM's or similar technology
• Code is written and works correctly
• Changes are covered by tests
• Any new configurable parameters are documented in rel/overlay/etc/default.ini
• Documentation changes were made in the src/docs folder
• Documentation changes were backported (separated PR) to affected branches