Refactoring HolidayProvider

[BelmoMusta]

In order not to repeat these instructions that are found in so many classes:

var holidays = HolidaySpecificationProcessor.Process(holidaySpecifications, countryCode);
return holidays.OrderBy(o => o.Date);

This logic is refactored in one abstract class AbstractHolidayProvider, which is inherited by other countries' providers

[tinohager]

@samtrion These are actually the changes we've already discussed, and I think they would suit me.

[samtrion]

The approach suits me and is complete with regard to the architecture tests.