Protobuf serializer

[peterychang]

Why are these changes needed?

Extend python serialization formats to include protobuf

Related issue number

https://github.com/microsoft/autogen/issues/3643

Checks

• [ ] I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• [ ] I've added tests (if relevant) corresponding to the changes introduced in this PR.
• [ ] I've made sure all auto checks have passed.