feat: Allows switching external playback mode in iOS

[jackwill99]

Description

Here are the specific behaviors when streaming to an Apple TV or Mac:

allowsExternalPlayback = true (default, current behavior of the audioplayers package)

• Both devices show fullscreen playback controls.
• On Apple TV, playback can be controlled with the remote, including seeking.
• Both devices become unusable for other tasks as the player covers the screen.

allowsExternalPlayback = false

• Apple TV shows a notification briefly when a new media item starts playing.
• macOS adds playback controls to the menu.

Apple Podcasts and other iOS apps use the allowsExternalPlayback = false setting, suggesting this is the intended behavior for pure audio apps. However, the video-like behavior with allowsExternalPlayback = true may still be useful for some developers as it enables playback controls on the Apple TV, even if this use case is rare.

Therefore, the default value is set to false.

Checklist

• The title of my PR starts with a [Conventional Commit] prefix (fix:, feat:, refactor:,
  docs:, chore:, test:, ci: etc).
• I have read the [Contributor Guide] and followed the process outlined for submitting PRs.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation and added dartdoc comments with ///, where necessary.
• I have updated/added relevant examples in [example].

Breaking Change

• Yes, this is a breaking change.
• No, this is not a breaking change.

Related Issues

[Gustl22]

Thank you for the contribution, and I'm really sorry, that it took so long to get to a review. The last months had been quite intense for me.
A few questions: So if I understand this correctly, this feature is about playing media on one device, but all other devices from the same ecosystem also allow controlling their playback, right?
Do you expect this feature to also be available on desktop platforms? Is there a similar feature on Android?
If not, then it would be a good candidate to be added to the AudioContext options:
https://github.com/bluefireteam/audioplayers/blob/main/getting_started.md#audio-context