Implement ext_proc header allow list feature (#27524)

* Implement ext_proc header scrubbing feature.

Signed-off-by: Yanjun Xiang <yanjunxiang@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ 547ef8858f71ce7e997b53eadf5fd75b32395ea6

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

@@ -187,7 +187,6 @@ message ExternalProcessor {
   // field is set in an external processor response.
   bool disable_clear_route_cache = 11;
 
-  // [#not-implemented-hide:]
   // Allow headers matching the ``forward_rules`` to be forwarded to the external processing server.
   // If not set, all headers are forwarded to the external processing server.
   HeaderForwardingRules forward_rules = 12;
@@ -209,6 +208,7 @@ message ExternalProcessor {
 // The HeaderForwardingRules structure specifies what headers are
 // allowed to be forwarded to the external processing server.
 message HeaderForwardingRules {
+  // If not set, all headers are forwarded to the external processing server.
   type.matcher.v3.ListStringMatcher allowed_headers = 1;
 }