proto: workaround for ECDS to be included in api/BUILD (#12367)

Signed-off-by: Lizan Zhou <lizan@tetrate.io>

Mirrored from https://github.com/envoyproxy/envoy @ edd20e56f0f6ba69cabd9b17a9ecdcf7c3bea37f

envoy/service/extension/v3/config_discovery.proto

@@ -36,3 +36,9 @@ service ExtensionConfigDiscoveryService {
     option (google.api.http).body = "*";
   }
 }
+
+// [#not-implemented-hide:] Not configuration. Workaround c++ protobuf issue
+// with importing services: https://github.com/google/protobuf/issues/4221 and
+// protoxform to upgrade the file.
+message EcdsDummy {
+}