Support setting response headers for DirectResponsePolicy Routes

[flawedmatrix]

I noticed that when I was trying to use a directResponsePolicy in a route, the responseHeadersPolicy would not actually take effect:

 routes:
  - conditions:
    - exact: /foo
    directResponsePolicy:
      statusCode: 200
    responseHeadersPolicy:
      set:
      - name: "Header-To-Add"
        value: "val"

$ curl -v https://example.site.com/foo
> GET /foo HTTP/2
> Host: example.site.com
> user-agent: curl/7.81.0
> accept: */*
...
< HTTP/2 200
< vary: Accept-Encoding
< date: Tue, 20 Feb 2024 23:43:58 GMT
< server: envoy

According to the Envoy docs, Direct Responses will include any headers specified in response_headers_to_add.

Perhaps this is a feature not yet added to Contour as response_headers_to_add is not populated in the case of a DirectResponse:

https://github.com/projectcontour/contour/blob/94e591fc8fab76c74a83d367b03e256062834ca5/internal/envoy/v3/route.go#L124-L125

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