Update the ContainerStoppedMetadata to be json serializable (facebookresearch#522)

Summary:
Pull Request resolved: facebookresearch#522

As title.

Reviewed By: danbunnell

Differential Revision: D45202654

fbshipit-source-id: 0b468a331f7e3a75e92254df1d70bb29ff80517b

Author: Michael Meng

fbpcp/entity/container_metadata.py

@@ -8,7 +8,10 @@
 
 from dataclasses import dataclass
 
+from dataclasses_json import dataclass_json
 
+
+@dataclass_json
 @dataclass
 class ContainerStoppedMetadata:
     stopped_at: str