add protobuf serialization test

[peterychang]

Why are these changes needed?

Add tests for python protobuf serialization

Related issue number

#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.

[jackgerrits]

Can you add a poe task gen-test-protos so its easy to regenerate them. Then extend this workflow to ensure it kept up to date:

autogen/.github/workflows/checks.yml

Line 171 in 908ed8c

poe gen-proto