Update EventHandler to use new client-go cache.ResourceEventHandler interface

[sunjayBhatia]

Let's try to take advantage of the new addition to the client-go ResourceEventHandler loop to make our processing of the initial resource list/startup better.

See the OnAdd method: https://github.com/kubernetes/client-go/blob/151a5919dbfe858f011422d6fcee2b998bb209d8/tools/cache/controller.go#L218

It adds a boolean parameter to signify whether the event comes as part of the initial List of resources on startup.

We can use this to improve our startup flow and ensure not just Informers are synced before we start the xDS server but that handlers actually receive the events.

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The Contour project currently lacks enough contributors to adequately respond to all Issues.

This bot triages Issues according to the following rules:

• After 60d of inactivity, lifecycle/stale is applied
• After 30d of inactivity since lifecycle/stale was applied, the Issue is closed

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[tsaarni]

xref #5672