FEAT: Add upload from url (instead of from disk) option

[deafmute1]

Is your feature request related to a problem? Please describe.
N/A

Describe the solution you'd like
Most of the time I have a book that is available over a public http(s) url that I have to download to disk then immediately upload to calibre-web. Would be nice (and hopefully/probably not too much effort) to be able to pass a url that calibre itself downloads.

adding this issue to track interest/potential implementation but I may submit a PR for it myself when I get time.

Describe alternatives you've considered
Currently works fine just extra step of downloading to disk.

Additional context
N/A