ext_authz: fix handling of non-UTF-8 bodies calling http_service (#28487)

Commit 664f3fce4730544f34ae767e10150fb6be11cdc6 changed how this data
is handled, but was only intended to apply when calling grpc_service.

Fixes #27386

Signed-off-by: Greg Greenway <ggreenway@apple.com>

Mirrored from https://github.com/envoyproxy/envoy @ 6b276066f4704abbbc870ed2bb71e3225476a1a2

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

@@ -208,6 +208,11 @@ message BufferSettings {
   // field of HTTP request attribute context. Otherwise, :ref:`body
   // <envoy_v3_api_field_service.auth.v3.AttributeContext.HttpRequest.body>` will be filled
   // with UTF-8 string request body.
+  //
+  // This field only affects configurations using a :ref:`grpc_service
+  // <envoy_v3_api_field_extensions.filters.http.ext_authz.v3.ExtAuthz.grpc_service>`. In configurations that use
+  // an :ref:`http_service <envoy_v3_api_field_extensions.filters.http.ext_authz.v3.ExtAuthz.http_service>`, this
+  // has no effect.
   bool pack_as_bytes = 3;
 }