Add optional return data for traces (#1)

* Add optional return data for traces

* Add explanation for non-requirement

Author: wmitsuda

src/schemas/ots-types.yaml

@@ -429,3 +429,7 @@ TraceEntry:
     input:
       title: calldata of the operation
       $ref: '#/components/schemas/bytes'
+    output:
+      title: return data of the operation
+      description: this value is optional for backward compatibility with old implementations that don't support it
+      $ref: '#/components/schemas/bytes'