Extracts a python value from an argument value proto.

arg_proto The proto containing a serialized value.
arg_function_language The arg_function_language field from Program.Language.
required_arg_name If set to None, the method will return None when given an unset proto value. If set to a string, the method will instead raise an error complaining that the value is missing in that situation.

The deserialized value, or else None if there was no set value and required_arg_name was set to None.