Content proposal support

[defagos]

As a tvOS user I would like to have a nice continuous playback experience between items in a playlist.

Acceptance criteria

• A content proposal can be offered during closing credits.
• The content proposal UI can be customized with SwiftUI.
• The demo implements a content proposal experience on tvOS. For SRG SSR content providing closing credits information the proposal is made during the credits, otherwise at the end of playback.

Hints

• Official documentation.
• Successfully tested on 845/credits-support-investigation. What is required:
  • Adding next content proposal to items (possible based on item metadata when recreating player items during queue player queue sync).
  • Creating a content proposal view controller.
  • Attaching an AVPlayerViewDelegate to manage content proposal display.
  • (not checked but likely) Calling advanceToNext when approving the content proposal.

Tasks

• [x] Attach next content proposals to items.
• [ ] Add API to let users customize content proposal presentation.
• [x] Update playlist player on tvOS to just display the full-screen system UI.
• [ ] Implement content proposal layout in demo.

[defagos]

We implemented this feature partially but realized it is probably too half-baked for our needs:

• The content proposal API must be applied on the current item, mixing metadata from the next content (a bit awkward but still feasible).
• Some presentation information (e.g. acceptance delay), UI-related, is defined on AVPlayerItem.
• The player desired frame is setup and animated in a UIKit world, making it difficult to interface with custom SwiftUI interfaces. Layout guides are provided but they do not live in the same view hierarchy as the content proposal UI and would be difficult to align on in a SwiftUI world anyway. Hardcoded values would be required when defining a content proposal layout.
• The accept / reject behavior is half-baked and inconsistent. For example the accept delegate method is not called when accepting automatically and the automatic acceptance date is defined both on items and in the UI, which is awkward.
• Content proposal dismissal APIs would need to be somehow exposed to SwiftUI so that they can be called.
• Automatic acceptance works only with AVPlayer, which stops when playback ends. With AVQueuePlayer playback would need to be paused as well. It seems that the content proposal was designed for use with AVPlayer, not AVQueuePlayer.

For all these reasons delivering a nice SwiftUI-friendly API is difficult. We'll therefore not implement this feature for the moment.