Add webp support
It would be great to use WebP (or even AVIF) for the generated avatar icons by appending the file extension as a suffix in the URL.
Then the website that uses Libravatar can choose if they want it in PNG or WebP format.
https://www.libravatar.org/static/img/nobody/80.png -- works today
https://www.libravatar.org/static/img/nobody/80.webp -- does not work