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.
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