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.

https://caniuse.com/webp

https://caniuse.com/avif

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

https://www.libravatar.org/avatar/23463b99b62a72f26ed677cc556c44e8?s=80 -- works today

https://www.libravatar.org/avatar/23463b99b62a72f26ed677cc556c44e8.webp?s=80 -- does not work