Handle overriding loading attribute in embedded media

Problem/Motivation

In #3247795-61: Add text filter plugin to support <img loading="lazy"> and remove it from editor_file_reference (and elsewhere in that issue) it was asked how to essentially override whatever the media view mode configuration says to do for loading of an image. If the image or responsive image field formatter is configured to render as eager or lazy, we might still want to override that setting in CKEditor.

<drupal-media> is embedding media (which might contain an image, but isn't just the image), with a defined view mode.

The media display mode configuration will include whether the image is rendered lazily or not (responsive image formatter still needs #2947796: Responsive image format for media.

If we do this issue and the others, but no specific follow-up, then it looks like this:

- If you add an image directly in the editor, you either specify the loading attribute, or this filter makes it lazy if nothing is specified.
- If you add an image via media embed, the media view display configuration determines the loading attribute

The responsibility for the loading attribute (and everything else about how the embedded media is displayed) is therefore on the media display configuration. This means that care needs to be taken about where any particular media view mode is likely to be used, since it may or may not appear above or below the fold, but that is the case for non-editor media references too.

There seem to be a couple of other options:

1. Make this filter apply the same change to images rendered via media embed too - however since every image rendered via media view mode will have a loading attribute of either lazy or eager, it would be a no-op.

2. Add configuration on media embed to specify the loading attribute , as mentioned in #3247795-21: Add text filter plugin to support and remove it from editor_file_reference, this would have to rewrite the rendered media entity HTML to override the attribute.

3. Take an approach closer to #2947796: Responsive image format for media thumbnails: allow the media reference to show media as a 'thumbnail' instead of with a view mode, and then make the image style/responsive image style and loading attribute configurable directly in the editor or something like that. This would have to be an option in the media embed widget in that case, although that allows the loading attribute to be unspecified, then this filter could then kick in on those images.

Option #2. seems possible, but counter-intuitive to have the rendered entity HTML rewritten by the filter.

Option #3 seems like it might be viable, although it adds a lot of options and complexity to the media embed widget.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet