introduce separate config flag internal-gitlab-server

This parameter is used exclusively for API requests. Thus we can route API traffic through an internal load balancer while still sending auth redirects though the front door (i.e. cloudflare).

Relates to gitlab#215321 (closed)

Refs https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/9982

cc @vshushlin @krasio @aamarsanaa @grzesiek @jhampton @sean_carroll @cmiskell @aamarsanaa @mwasilewski-gitlab @msmiley @jarv