Add Fluent API / Builder Pattern

[tavanuka]

• Current issue: Multiple overloads with many optional parameters
  potential improvement:

await client.QueryHolidays()
    .ForYear(2024)
    .InStates(GermanState.Bavaria, GermanState.Berlin)
    .IncludeCatholic()
    .ExecuteAsync();

Pros: More flexible API, functional approach and customizability