Load images lazily

[defagos]

As a developer integrating Pillarbox I don't want images to be unnecessarily loaded if not used, e.g. if no chapters are displayed in my custom user interface.

Acceptance criteria

• The main artwork image is not loaded if not used (e.g. layout, Control Center or system UI).
• Chapter images are not loaded if not used (custom chapter UI or system UI).

Tasks

• [x] Introduce lazy loading mechanism for images.
• [x] Do not attach artwork images to external metadata on iOS (not used).
• [x] Improve test suite (we can likely test player metadata changes instead of now playing updates, and only test mapping to now playing info once).
• [x] Build now playing info at the very last moment so that images are loaded only if needed.
• [x] Introduce UIImage helper and LazyImage view for lazy image loading in custom UIs.
• [ ] ~Avoid ugly lazy image animations when transitioning between contents whose artworks have different aspect rations (e.g. Forum to Vertigo transition in audio playlist example).~
• [x] Increase artwork image resolution for the IL data source.
• [x] Check behavior with a proxy. If images are not used (e.g. no Control Center, no standard system UI integration, no chapters), then they must no appear in the network traffic.

[defagos]

AVMetadataItem, even though its API seems to support lazy loading, calls its value loading handler in a not-so-lazy way. Even if a metadata item, assigned to the externalMetadata of an AVPlayerItem, is not used in any way (e.g. no AVPlayerViewController is used), the handler will namely be called.

For this reason it appears we cannot really lazily load the main artwork image associated with an item, as it needs to be stored in its externalMetadata. For this reason it is also not useful to lazily load the artwork in the now playing info.