Provide std::format support for Q(Date|Time)+ classes

C++20 already provides a bunch of std::formatter specializations for chrono types.
See the list of supported overloads here. Each formatter specialization defines a set of supported format specifiers.

QDateTime already support conversion to std::chrono::sys_time, so we can just use the respective formatter overload.

QDate is convertible to std::chrono::sys_days, and we probably need to convert it to std::chrono::year_month_day and use the respective formatter.

QTime does not have any conversions to std::chrono types, but std::chrono::hh_mm_ss and the related formatter look like a reasonable choice.