Image Card: Allow for custom aspect ratio in collection variant

[daneah]

The problem

I want to use the collection variant of the image card to get its showy heading style, but I also have images that aren't 4:3.

The solution

Be able to specify the aspect ratio of the image explicitly instead of hard coding it, or better, allow the card to shrink naturally like it does for the default variant.

Alternatives considered

We could use the default variant to shrink to the image's natural aspect, but it doesn't have a showy heading style. We could create new images that are 4:3, but that doesn't track well.

Additional information

It may be that the "collection" variant is a bit of a tightly-scoped name as well—it's a bit of a "showcase" variant that may not always be showcasing a collection per se. That's really a semantics issue, but good to have language that can be used ubiquitously.