Skip external images

Problem/Motivation

When a external image link is in a content, webp_extras/src/Plugin/Filter/WebpImage.php:process() method try to convert it to webp and fail. It is causing an error:

Could not generate image resource from URI
public://https://fastly.picsum.photos/id/901/200/300.jpg

Note: https://fastly.picsum.photos/id/901/200/300.jpg is just a example.

Proposed resolution

Skip process when image URL is external.