Implemented the ability to update a library including folders.

API/DTOs/UpdateLibraryForUserDto.cs

@@ -0,0 +1,10 @@
+using System.Collections.Generic;
+
+namespace API.DTOs
+{
+    public class UpdateLibraryForUserDto
+    {
+        public string Username { get; init; }
+        public IEnumerable<LibraryDto> SelectedLibraries { get; init; }
+    }
+}