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 se han identificado archivos, pruebas ni puntos de entrada. Empieza rastreando la publicación de artículos y el flujo existente de carga de imágenes; después, define cómo se detectan y almacenan las imágenes externas; se considera terminado cuando el contenido publicado sustituye esos enlaces por imágenes alojadas en el sitio y se cubre el escaneo solicitado de todo el sitio.

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