Add protobuf json serialization support to MesopJSONEncoder

[bamit5]

Is your feature request related to a problem? Please describe.
I'm unable to use protobufs in Mesop's state since they're not json serializable. This is frustrating since Google's internal tools are built on protobufs.

Describe the solution you'd like
Add protobuf.json_format support for protobufs in MesopJSONEncoder.

Describe alternatives you've considered
Haven't considered other solutions. This seems like intuitively the best solution.

Additional context
N/a

[richard-to]

That's a fair point. If all we need to do is call that function, then I think we can try to add support for protobufs in Mesop State. I'll give it a try when I get the chance.