Updating ext_proc filter clear route cache logic. (#26388)

Adding in the ability to disable route cache clearing ont he ext_proc filter. Changing the behavior of the clearing to check if there is a header mutation.

Risk Level: Low
Testing: test/extensions/filters/http/ext_proc/filter_test.cc
Docs Changes: docs/root/configuration/http/http_filters/ext_proc_filter.rst
Release Notes: changelogs/current.yaml

Signed-off-by: Joseph Straceski <jstraceski@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ a3f01a5190b56e576842ac3b08b337c181a02bda

envoy/extensions/filters/http/ext_proc/v3/ext_proc.proto

@@ -95,7 +95,7 @@ option (xds.annotations.v3.file_status).work_in_progress = true;
 // messages, and the server must reply with
 // :ref:`ProcessingResponse <envoy_v3_api_msg_service.ext_proc.v3.ProcessingResponse>`.
 
-// [#next-free-field: 11]
+// [#next-free-field: 12]
 message ExternalProcessor {
   // Configuration for the gRPC service that the filter will communicate with.
   // The filter supports both the "Envoy" and "Google" gRPC clients.
@@ -168,6 +168,11 @@ message ExternalProcessor {
   // :ref:`override_message_timeout <envoy_v3_api_field_service.ext_proc.v3.ProcessingResponse.override_message_timeout>`
   // If not specified, by default it is 0, which will effectively disable the ``override_message_timeout`` API.
   google.protobuf.Duration max_message_timeout = 10;
+
+  // Prevents clearing the route-cache when the
+  // :ref:`clear_route_cache <envoy_v3_api_field_service.ext_proc.v3.CommonResponse.clear_route_cache>`
+  // field is set in an external processor response.
+  bool disable_clear_route_cache = 11;
 }
 
 // Extra settings that may be added to per-route configuration for a