Python wrappers that return protobuf serialized strings sometimes fail when decoded as utf-8

[Aposhian]

When a C++ function returns a std::string that is a serialized protobuf (like in callService), and that is part of a Pybind11 wrapper, Pybind11 assumes it is UTF-8 and will try to decode it as such.

When a C++ function returns a std::string or char* to a Python caller, pybind11 will assume that the string is valid UTF-8 and will decode it to a native Python str, using the same API as Python uses to perform bytes.decode('utf-8'). If this implicit conversion fails, pybind11 will raise a UnicodeDecodeError.

https://pybind11.readthedocs.io/en/stable/advanced/cast/strings.html#returning-c-strings-to-python

This is not a valid assumption for protobuf, and so sometimes an exception is raised when returning the service response, and so you might see an exception like:

'utf-8' codec can't decode byte in position 14 invalid start byte '0x3'

The fix is to instead have the python wrappers return py::bytes which will be directly returned as a bytes object in Python without any attempt to decode.