ExtensionService should support requestHeadersPolicy

**Please describe the problem you have**
[A clear, concise, description of the problem you are facing. What is the _problem_ that feature X would solve for you?]

I'm trying to set `requestHeadersPolicy` for an `ExtensionService` in order to set the `Host` header. This is important in 2 scenarios for me:
1. Using an ExternalName service for deployments in Google Cloud Run with ExtensionService that requires the Host header to be rewritten
2. Using a service with Knative Serving where Host headers need to be included in the request

Is there any way to do this now? It would be great to have ExternalAuthorization work against a DNS that's outside the cluster or within the cluster but running on Knative Serving.

Start by tracing Contour's Kubernetes ExtensionService and ExternalAuthorization entry points, focusing on how requestHeadersPolicy is represented. Confirm the configuration needed to rewrite the Host header for external DNS and Knative Serving services; done means those authorization targets can receive the required Host header.

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