[Cascader] Asynchronous loading supports default values

[snowchenlei]

• I have searched the issues of this repository and believe that this is not a duplicate.

What problem does this feature solve?

当前版本：2.2.2
重新链接：https://stackblitz.com/edit/vue-urjsjw?file=src/App.vue
1、Cascader使用异步加载数据(并默认加载了第一层)。设置defaultValue后只能默认加载第一层，不会触发change事件也无法异步加载默认值，希望可以支持异步数据默认值。
2、支持选项改变时触发事件。change仅在selected或整个值变化是才触发，设置了第一层值并不会触发
change在选项改变时应该触发

What does the proposed API look like?

(参考ng.antd)
https://ng.ant.design/components/cascader/zh#components-cascader-demo-default-value-and-lazyload

[tangjinzhou]

https://2x.antdv.com/docs/vue/faq-cn
当我动态改变 defaultValue,defaultOpenKeys, initialValue等defaultXxxx的时候它并没有生效。
Input/Select 等的 defaultXxxx（例如 defaultValue）只有在组件第一次渲染的时候有效，此特性参考自React。切记：第一次、第一次、第一次....

[snowchenlei]

如果不用defaultXXX，直接使用v-modal:value也不能显示默认值。
请问异步加载情况下如何才能显示默认值呢？

[github-actions]

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