Feature Request: Support for multiple image URLs for more efficient loading of high/low res images

Most modern apps use various resolutions of the same image throughout the app. For example, a low res image for a thumbnail and a full res image for viewing one image at a time. Often this means that we are loading and then caching two copies of the same image. It would be pretty helpful to be able to pass a high and low res image in, use the high res image if it exists in the cache, and then fallback to the low res image if it does not (from cache or network).

Alternatively, it would also be helpful to expose an easy way to check if an item is in the cache so that the consumer can do this work if they choose. I imagine it is technically possible to do this with `flutter_cache_manager`, but it would be great to expose it in the public API of `flutter_cached_network_image`.