fairlogger v2.0.0 changes return type of some functions

[tntakahashi]

In version 2.0.0 of fairlogger, some functions return std::string_view instead of std::string.
This change causes the compilation errors with TelemetryPlugin.cxx

    (opt::TelemetrySeverity.data(), bpo::value<std::string>()->default_value(fair::Logger::SeverityName(fair::Severity::trace)),

The above line should be

    (opt::TelemetrySeverity.data(), bpo::value<std::string>()->default_value(fair::Logger::SeverityName(fair::Severity::trace).data()),