Drawio - Add resolution of external image to get self contained SVG

[jynolen]

This PR help to keep drawio svg self contained.

Background I use SelfHosted Kroki with antora doc generator When using drawio diagrams it happned that some part of the diagr was missing because it relies on external images.

What have been done:

With this PR, during the rendering phase, kroki will retreive external image to embed them as base64 src. This way diagrram will be rendered correctly whenever and wherever