Load chapters and artwork images lazily

[waliid]

Description

This PR allows us to load chapters and artwork images only when these are used.

Changes made

• Introduces NowPlaying and replaces NowPlayingInfo by NowPlaying.Info
• The now playing info are merged only when the info property is accessed.
• Introduces ImageSource.
• The PlayerMedatadata uses an image source instead of using an image.
• Adds LazyImage to load lazily an image from a given image source.
• Adds LazyUIImage to load lazily an UIImage from a given image source.

Checklist

• APIs have been properly documented (if relevant).
• The documentation has been updated (if relevant).
• New unit tests have been written (if relevant).
• The demo has been updated (if relevant).