Enhancement: Allow excluding images from Photon by class name.

### Impacted plugin

Jetpack

### What

If I add a specific class to any image (e.g. 'do-not-photonize') then that image will not be served by Photon.

### How

@jeherve looked into this briefly, but could not figure out how to add a class to the image tag itself (only the figure class) using the block editor.

Start by tracing Jetpack's Photon image handling and the WordPress block editor path that adds classes to image and figure tags. Confirm whether the issue is still applicable, then determine how an image class such as `do-not-photonize` can exclude that image and verify the behavior with the relevant existing tests or a block-editor image reproduction.

Written by the indexing model from the issue text.