[gw api] discover certs for route hostnames

[zac-nixon]

Issue

#4391

Description

The Ingress API will discover certs for all host names, on the listener and each rule. We missed that for the Gateway API, so I am back porting the same logic.

Checklist

• Added tests that cover your change (if possible)
• Added/modified documentation as required (such as the README.md, or the docs directory)
• Manually tested
• Made sure the title of the PR is a good description that can go into the release notes

BONUS POINTS checklist: complete for good vibes and maybe prizes?! 🤯

• Backfilled missing tests for code in same general area 🎉
• Refactored something and made the world a better place 🌟

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: zac-nixon

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[shuqz]

/lgtm