Rename `Asset.image` to `Asset.img`.

[alightr]

The Asset object defines image asset media under the image property. This is a mismatch against the naming convention used in the AssetFormat object as well as the Asset definition inside the native ads spec [1], which define the image format inside the img property.

[1] https://www.iab.com/wp-content/uploads/2018/03/OpenRTB-Native-Ads-Specification-Final-1.2.pdf