feat: Support route fallback by default (#1381)

helm/core/templates/fallback-envoyfilter.yaml

@@ -0,0 +1,22 @@
+apiVersion: networking.istio.io/v1alpha3
+kind: EnvoyFilter
+metadata:
+  name: {{ include "gateway.name" . }}-global-custom-response
+  namespace: {{ .Release.Namespace }}
+  labels:
+    {{- include "gateway.labels" . | nindent 4}}
+spec:
+  configPatches:
+    - applyTo: HTTP_FILTER
+      match:
+        context: GATEWAY
+        listener:
+          filterChain:
+            filter:
+              name: envoy.filters.network.http_connection_manager
+      patch:
+        operation: INSERT_FIRST
+        value:
+          name: envoy.filters.http.custom_response
+          typed_config:
+            "@type": type.googleapis.com/envoy.extensions.filters.http.custom_response.v3.CustomResponse