add option to prefer directlinks

ModpackUpdater/ModpackConfig.cs

@@ -10,6 +10,7 @@ public class ModpackConfig
     public string InstallUrl { get; set; }
     public string UnleashApiUrl { get; set; }
     public string UnleashInstanceId { get; set; }
+    public bool PreferDirectLinks { get; set; }
 
     [JsonIgnore]
     public string ConfigUrl { get; set; }