Ensure the wrapper does not fills 'allowed_sasl_mechanisms' field unless User does it explicitly

Since CXXCBC-296, default SASL mechanism selection has been moved down to Core, so unless the user explicitly uses ldapCompatible constructor for PasswordAuthenticator (the one that resets allowed mechanisms to ['PLAIN']), or specifies SASL list of mechanisms explicitly, the wrapper should not fill allowed_sasl_mechanisms for C++SDK.