Add support for ExternalAuth

[salonichf5]

One of our customers wants to use NGINX Gateway Fabric to authenticate requests via an external authorization proxy before routing traffic to backend applications. This requires adding support for NGINX’s http_auth_request module to enable external authentication checks.

Acceptance

• Add support for [HTTPExternalAuthFilter](https://gateway-api.sigs.k8s.io/reference/spec/#httpexternalauthfilter
• Document how to use this feature (could be combined with our own AuthFilter doc)

[nginx-discourse-bot]

This issue has been mentioned on NGINX Community Forum. There might be relevant details there:

https://community.nginx.org/t/traffic-split-for-k8s-dashboard-using-oauth2-proxy-using-ngf/8600/2

[Nikoolayy1]

I also see this as a great option to have!

[github-actions]

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