ext_proc: Add per-route configuration (#17691)

Today "disabled" and the processing mode are the two things supported
because that is what is currently supported in ext_proc.

Signed-off-by: Gregory Brail <gregbrail@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ 01ea37b280d980c25bb9e2c8069d62e4aef37961

envoy/extensions/filters/http/ext_proc/v3alpha/ext_proc.proto

@@ -150,7 +150,6 @@ message ExternalProcessor {
   string stat_prefix = 8;
 }
 
-// [#not-implemented-hide:]
 // Extra settings that may be added to per-route configuration for a
 // virtual host or cluster.
 message ExtProcPerRoute {
@@ -161,23 +160,27 @@ message ExtProcPerRoute {
     // If disabled is specified in multiple per-filter-configs, the most specific one will be used.
     bool disabled = 1 [(validate.rules).bool = {const: true}];
 
-    // Override aspects of the configuration for this route
+    // Override aspects of the configuration for this route. A set of
+    // overrides in a more specific configuration will override a "disabled"
+    // flag set in a less-specific one.
     ExtProcOverrides overrides = 2;
   }
 }
 
-// [#not-implemented-hide:]
 // Overrides that may be set on a per-route basis
 message ExtProcOverrides {
   // Set a different processing mode for this route than the default.
   ProcessingMode processing_mode = 1;
 
+  // [#not-implemented-hide:]
   // Set a different asynchronous processing option than the default.
   bool async_mode = 2;
 
+  // [#not-implemented-hide:]
   // Set different optional properties than the default.
   repeated string request_properties = 3;
 
+  // [#not-implemented-hide:]
   // Set different optional properties than the default.
   repeated string response_properties = 4;
 }