Support forwarded headers with external auth

[arsnyder16]

A way to allow forwarded headers to be passed to external auth configured with auth-url.

Currently when using auth-url despite the values of use-forwarded-headers and compute-full-forwarded-for only x-forwarded-for is sent to the external auth service. x-forward-proto, x-forwarded-scheme etc are stripped. Another set of configurations similar to use-forwarded-headers and compute-full-forwarded-for that apply to just the external auth would be desirable.

Currently a work around is to use auth-proxy-set-headers and hardcode any value, but this has it limitations since it isn't dynamic

[k8s-ci-robot]

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[github-actions]

This is stale, but we won't close it automatically, just bare in mind the maintainers may be busy with other tasks and will reach your issue ASAP. If you have any question or request to prioritize this, please reach #ingress-nginx-dev on Kubernetes Slack.