api: add handling for xds.type.v3.TypedStruct (#18567)

Signed-off-by: Mark D. Roth <roth@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ 8af9f31db2f96721aa3c94265c66d7140f6bce95

envoy/config/core/v3/extension.proto

@@ -24,8 +24,9 @@ message TypedExtensionConfig {
   string name = 1 [(validate.rules).string = {min_len: 1}];
 
   // The typed config for the extension. The type URL will be used to identify
-  // the extension. In the case that the type URL is *udpa.type.v1.TypedStruct*,
+  // the extension. In the case that the type URL is *xds.type.v3.TypedStruct*
-  // the inner type URL of *TypedStruct* will be utilized. See the
+  // (or, for historical reasons, *udpa.type.v1.TypedStruct*), the inner type
+  // URL of *TypedStruct* will be utilized. See the
   // :ref:`extension configuration overview
   // <config_overview_extension_configuration>` for further details.
   google.protobuf.Any typed_config = 2 [(validate.rules).any = {required: true}];