use one instance of acm cert discovery in gateway builder

[zac-nixon]

Description

While testing the Gateway API implementation at scale, I noticed a tiny memory leak (90Mi -> 110Mi) over the course of a weekend. I believe the issue is that we're recreating the cert discovery component for every build listener call in the Gateway API. This object is meant to be a singleton, as it creates caches on every instantiation. As a bonus, this seems to reduce the latency quite considerably for reconciliation times:

The screenshot shows reconcile runs that finish in under 10 seconds [I have 200 gateways provisioned]. The first line is with using 1 cert discovery instance per listener builder, the following lines are using the singleton.

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 🌟

[shuqz]

/lgtm
/approved

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: wweiwei-li, zac-nixon

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