fix: compilation on Windows
The utility::string_t used by the AzureStorage backend is a typedef for std::wstring defined in cpprestsdk: https://github.com/microsoft/cpprestsdk/blob/master/Release/include/cpprest/details/basic_types.h#L36 On Windows std::wstring is a 2 byte wide utf16 character stream. To securly convert between std::wstring and std::string on Windows a utf16 -> utf8 routine has be applied. Also there are no implecit conversions from std::wstring and std::string.