Add backgroundImage prop and support image-set

Sometimes Image component is not enough to address Design requirements and we need backgroundImage CSS prop to set a background image to a Box. However, this prop loads the source image without optimizations. Image component leverages srcset to optimize source asset from provider thus loading best image quality with smaller file size. BackgroundImage supports a similar strategy with image-set, which has been supported by major browsers for a while. It would be great to have that optimization.