Improve type of Image on front card

As a follow up from #8228, we should improve the image type coming through the `enhanceCards` and `enhanceCollections` enhancers.

We should have one common `Image` type which includes `src` and `alt` as a minimum (see description on #8228)

This came up recently in [this PR](https://github.com/guardian/dotcom-rendering/pull/8860) which suggests a sensible fallback option for missing alt text, though this PR does not solve the underlying issue of the data not coming through