[AiChat] Add reference sources of AI response

[yucaowang]

这个功能解决了什么问题

Most RAG based chatbot will show the reference documents or source URLs to users for further information, but currently the ChatItem does not have component to show the reference sources.

你建议的方案是什么

Add a 'sources' field for ChatItem, it could be array of reference sources and each source are clickable.

[github-actions]

👋 @yucaowang，感谢给 TDesign 提出了 issue。
请根据 issue 模版确保背景信息的完善，我们将调查并尽快回复你。

[zydemail]

可以使用chat-item的slot插槽来实现，content插槽内容支持自定义，你可以把引用来源放在content插槽里来写
https://tdesign.tencent.com/chat/components/chat-item

实现效果如官网所示：