Supersedes: #34500

Also this will allow to fix this: #35386 without the UX changes but
providers can now be control through settings as well within zed.

Just rebased the latest main and docs added. Added @AurelienTollard as
co-author as it was started by him everything else remains the same from
original PR.

Release Notes:

- Added ability to control Provider Routing for OpenRouter models from
settings.

Co-authored-by: Aurelien Tollard <tollard.aurelien1999@gmail.com>