Resolve HA / multi-thread concerns around Gateway updates

[evankanderson]

Currenty, we assume a single Gateway resource; when using auto-TLS, we need to update Spec.Listeners for each Knative Route.

The default controller implementation will attempt to reconcile multiple KIngress resources at once, which means that it's likely that we will get some number of update collisions on the Gateway. We should figure out a mechanism for spreading updates across multiple Gateways to enable HA / multi-threaded controllers without bottlenecking on a single Gateway instance.

This is extracted from this PR comment

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[evankanderson]

/remove-lifecycle stale

I'll take a look and see if this is still necessary

[dprotaso]

Sorta a dupe of #363

Two potential options to explore:

1. From It is hard to programmatically manage `Spec.Listeners` kubernetes-sigs/gateway-api#1248 - add a conformance test to assert Gateway merges and push for this to be part of the CORE.
2. See if Server side apply gives us some granularity here - maybe we use distinct field managers for listener entries - though this implies there isn't some validation preventing duplicate listeners (which is trying to be added via Webhook: Validate unique port, protocol, and hostname per listener kubernetes-sigs/gateway-api#847) which I'm trying to halt - Webhook: Validate unique port, protocol, and hostname per listener kubernetes-sigs/gateway-api#847 (comment) (ignore as hostname would be unique in each listener)

Related to 1. I think we should formalize a different way of merging Gateways beyond having the same addresses. I'll make a GEP for this.

[dprotaso]

Upstream GEP issue
kubernetes-sigs/gateway-api#1713

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