Add an API for retrieving image thumbnails

[kean]

Example usage:

let thumbnail = ImageRequest.ThumbnailOptions(maxPixelSize: 400)
let request = ImageRequest(url: URL(string: "http://..."), userInfo: [.thumbnailKey: thumbnail])

Documentation: https://kean.blog/nuke/guides/customizing-requests#thumbnails

Note: fully supports caching, task coalescing, disables decompression (already decompressed).