Support for Artwork

Heya!

Thanks for this great package. I was looking though, and I noticed that it doesn't have any support for artwork at the moment.

I was looking into this, and I noticed that the Android SDK and iOS SDK handle artwork pretty differently (is this why you didn't bother implementing it? understandable, if so).

For Android, it's a simple [field](https://spotify.github.io/android-sdk/app-remote-lib/docs/index.html), and it is available for only tracks.

For iOS, it is also only available for tracks. You have to use a [getter](https://spotify.github.io/ios-sdk/html/Protocols/SPTAppRemoteImageAPI.html) that accepts an [interface](https://spotify.github.io/ios-sdk/html/Protocols/SPTAppRemoteImageRepresentable.html) that only [tracks](https://spotify.github.io/ios-sdk/html/Protocols/SPTAppRemoteTrack.html) seem to implement. [Example](https://github.dev/spotify/ios-sdk/blob/447dfa25ad36abf145d7048f605702f4bd6e9821/DemoProjects/SPTLoginSampleAppSwift/SPTLoginSampleAppSwift/ViewController.swift#L143-L152).

I'm just wondering if you still plan on working on this eventually. It's no biggie if you aren't, since people can just revert to using something like [this package](https://github.com/Nishant1500/Spotify-Cover-Art-API).

The way I imagined this is making it a promise that resolves immediately for Android bur is actually async for ios (since it actually has to do fetching).