update button(FormItem)

因该类中  type = “button" 在校验中无法通过，需修改为
  type: str = "button"，增加参数验证

fastapi_amis_admin/amis/components.py

@@ -790,7 +790,7 @@ class InputSubForm(FormItem):
 class Button(FormItem):
     """Button"""
 
-    type = "button"
+    type: str = "button"
     className: str = None  # Specify the add button class name
     url: str = None  # Click the jump address, specify the behavior of this attribute button is consistent with the
     # a link