feat: support dynamic form group generation, add demo.

demo/Ursa.Demo/DataTemplates/FormDataTemplateSelector.cs

@@ -0,0 +1,24 @@
+using Avalonia.Controls;
+using Avalonia.Controls.Templates;
+using Ursa.Demo.ViewModels;
+
+namespace Ursa.Demo.Converters;
+
+public class FormDataTemplateSelector: ResourceDictionary, IDataTemplate
+{
+    public Control? Build(object? param)
+    {
+        if (param is null) return null;
+        var type = param.GetType();
+        if (this.TryGetResource(type, null, out var template) && template is IDataTemplate dataTemplate)
+        {
+            return dataTemplate.Build(param);
+        }
+        return null;
+    }
+
+    public bool Match(object? data)
+    {
+        return data is IFromItemViewModel;
+    }
+}