Add ResponseHeaderModifier filter (#62)

As in 

https://gateway-api.sigs.k8s.io/references/spec/#gateway.networking.k8s.io/v1beta1.HTTPRouteFilter

and

https://github.com/kubernetes-sigs/gateway-api/blob/main/apis/v1beta1/httproute_types.go#L615

Signed-off-by: Alex Leong <[email protected]>

Author: adleong

src/httproute.rs

@@ -361,6 +361,15 @@ pub enum HttpRouteFilter {
         request_header_modifier: HttpRequestHeaderFilter,
     },
 
+    /// ResponseHeaderModifier defines a schema for a filter that modifies
+    /// response headers.
+    ///
+    /// Support: Extended
+    #[serde(rename_all = "camelCase")]
+    ResponseHeaderModifier {
+        response_header_modifier: HttpRequestHeaderFilter,
+    },
+
     /// RequestMirror defines a schema for a filter that mirrors requests.
     /// Requests are sent to the specified destination, but responses from
     /// that destination are ignored.