Implement header forwarding disallow list feature for ext_proc filter. (#28484)

* Add header forwarding disallow list support for ext_proc filter.

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

Mirrored from https://github.com/envoyproxy/envoy @ 852326772e76621cb495b38cec571d60ac8493b5

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

@@ -221,7 +221,6 @@ message HeaderForwardingRules {
 
   // If set, specifically disallow any header in this list to be forwarded to the external
   // processing server. This overrides the above ``allowed_headers`` if a header matches both.
-  // [#not-implemented-hide:]
   type.matcher.v3.ListStringMatcher disallowed_headers = 2;
 }