[Feature]: Filtering & LINQ extensions

[tavanuka]

• Current Issue: API supports multiple years/states but no convenience methods
  ###Improvement:
• GetHolidaysForYearRange(startYear, endYear)
• GetAllGermanHolidays(year) - fetches all states

User Benefit: Common scenarios simplified

after #8 has been concluded, evaluate if this is something that should be worked on