Make trusting X-Forwarded-For headers configurable
Making a public MR as this is a low-severity issue.
This allows us to opt out of trusting X-Forwarded-For headers. We'll want that for pages, as it terminates TLS on its own, and also has its own XFF handling internally.
refs https://gitlab.com/gitlab-org/gitlab-pages/-/issues/525.
cc @vshushlin