Compatibility with Protobuf 4.XX

Hi! Our team is very interested in using this package, but the lack of 4.X protobuf support is blocking us pulling it into our repo (todos in a few setup.py [ex](https://github.com/google/fhir-py/blob/main/google-fhir-core/setup.py#L106)).

```
# TODO(b/276635321): Fix compatibility issue with protobuf 4.x
```

Curious if there's any chance this will be addressed soon / if the incompatibilities could be elaborated on, we'd be willing to contribute.

Thanks!