Automatically replace the external links of pictures in the content with those of your own website

**Is your feature request related to a problem? Please describe.**
Currently, whether a local markdown file or an article reprinted from other platforms, the image links inside will not be replaced with the website's own links, which may lead to the risk of image failure.

**Describe the solution you'd like**
I want a function that when publishing an article, I can automatically download and upload external link-type pictures to my own website server, and automatically replace them with pictures from my own server in the article.
I also want a plug-in that can automatically scan all the external link images in my website and do the above.

**Describe alternatives you've considered**
Considering that when pasting a picture, it will automatically upload the picture and write the address of the picture into the content, I think this function should not be difficult to implement.

尚未确定任何文件、测试或入口点。首先跟踪文章发布流程和现有的图片上传流程，然后定义如何检测和存储外部图片；当已发布内容将这些链接替换为由站点托管的图片，并且涵盖所请求的全站扫描时，即视为完成。

由索引模型根据 Issue 内容生成。