Added initial models.

Harmonia.Core/Models/SortOption.cs

@@ -0,0 +1,9 @@
+using System.ComponentModel;
+
+namespace Harmonia.Core.Models;
+
+public class SortOption(string fieldName, ListSortDirection direction)
+{
+    public string FieldName { get; init; } = fieldName;
+    public ListSortDirection Direction { get; init; } = direction;
+}