Add response_header_modifier outbound http filter (#251)

The `responseHeaderModifier` filter is an Extended level feature:

https://gateway-api.sigs.k8s.io/references/spec/#gateway.networking.k8s.io%2fv1beta1.HTTPRouteFilter

We add this filter to the outbound proxy-api.

Signed-off-by: Alex Leong <alex@buoyant.io>

Author: adleong

go/http_route/http_route.pb.go

@@ -84,6 +84,24 @@ message RequestHeaderModifier {
     repeated string remove = 3;
 }
 
+// Configures a route to modify a response's headers.
+//
+// Modifications are to be applied in the order they are described here:
+// additions apply first, then sets, and then removals.
+message ResponseHeaderModifier {
+    // A list of headers name-value pairs to set on responses, augmenting any
+    // existing values for the header.
+    io.linkerd.proxy.http_types.Headers add = 1;
+
+    // A list of headers name-value pairs to set on responses, replacing any
+    // existing values for the header.
+    io.linkerd.proxy.http_types.Headers set = 2;
+
+    // A list of headers names to be removed from responses.
+    repeated string remove = 3;
+}
+
+
 // Configures a route to respond with a redirect response. The `location` header
 // is set with the given URL parameters.
 message RequestRedirect {

go/outbound/outbound.pb.go

@@ -125,6 +125,7 @@ message HttpRoute {
       io.linkerd.proxy.http_route.HttpFailureInjector failure_injector = 1;
       io.linkerd.proxy.http_route.RequestHeaderModifier request_header_modifier = 2;
       io.linkerd.proxy.http_route.RequestRedirect redirect = 3;
+      io.linkerd.proxy.http_route.ResponseHeaderModifier response_header_modifier = 4;
     }
   }