Feature: Add attachment from URL (e.g. image link)

[cvlc12]

Is your feature request related to a problem? Please describe.
To attach an image to a card, you need to either upload it from local storage or link it from nextcloud files.

Describe the solution you'd like
It would be great to have a third option to specify a URL to a file or image, and this is uploaded and stored.

This would especially be handy to quickly point a card to an image url and have that associated with the card, without having to first download and store the image in nextcloud and thereafter use it!

thanks !