Add support for Live Photo loading from network

[onevcat]

This PR implements the requirement of #2297

It mainly adds a high level extension method to PHLivePhoto to load a live photo (with two URLs, one for still image, one for video). That means, you can now use this code to load a live photo easily:

let urls = [
  URL(string: "https://example.com/image.heic")!, // imageURL
  URL(string: "https://example.com/video.mov")!   // videoURL
]
let livePhotoView = PHLivePhotoView()
livePhotoView.kf.setImage(with: urls)

The still image and video data can be retrieved by PHAssetResourceManager.requestData and uploaded to the server by a user. Then in another client, these files can be loaded and it finally generates a PHLivePhoto object.

It is supported on all platforms where PHLivePhotoView is available (iOS, macOS, tvOS, etc).

For more detail, a following PR of documentation will be added soon.