Enhancement Proposal: Authentication Filter

[shaun-nx]

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Proposed changes

This document proposes a solution for enabling Authentication use cases through NGINX Gateway Fabric.

Closes https://github.com/nginx/nginx-gateway-fabric/issues/4052

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• [x] I have read the CONTRIBUTING doc
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have checked that all unit tests pass after adding my changes
• [x] I have updated necessary documentation
• [x] I have rebased my branch onto main
• [ ] I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

Release notes

If this PR introduces a change that affects users and needs to be mentioned in the release notes, please add a brief note that summarizes the change.

NONE