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.

No files, tests, or entry points are identified. Start by tracing article publishing and the existing image-upload flow, then define how external images are detected and stored; done means published content replaces those links with site-hosted images and the requested site-wide scan is covered.

Written by the indexing model from the issue text.