Add webp format for images

The webp format is not accepted as a variant in ActiveStorage by default so, although the system does not break if you upload an image in .webp format, it is not accepted either.
ActiveStorage does not have this extension by default to be able to create variants with this content type.
The proposal is to add this extension in ActiveStorage through an initializer like in this PR: https://github.com/diputacioBCN/decidim-diba/pull/135

Furthermore, since this extension is already accepted by most current browsers:  https://caniuse.com/webp

This format could be accepted when uploading images on the entire Decidim platform.

Here are some issues related to this topic:
https://github.com/decidim/decidim/issues/10126
https://github.com/decidim/decidim/issues/10136

[New]
The PR of the feature is here: https://github.com/decidim/decidim/pull/12383