Prefetch art URIs

[hacker1024]

At the moment (as far as I can tell), the art is only fetched when the song starts playing.
It'd be better if the plugin prefetched art for items in the queue, so there's no delay in showing the art in the notification.

[alexantenna]

Another option is to support paths to images in file system, in this case we may preload them by ourselves. Is it possible?

[hacker1024]

I don't want unnecessary I/O. There should be an option to preload in memory.

[ryanheise]

I've added a shouldPreloadArtwork parameter to AudioService.start which preloads artwork for items in the queue. Although this feature works, it seems to have exposed a block somewhere in the Flutter engine whereby any communication over platform channels is momentarily delayed while the preloading is in progress (despite this being done in a background thread). As such, this option defaults to false for now.

[github-actions]

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