Allow external audio session managers via a public protocol

[svenmeyers89]

While integrating the Readium SDK, I noticed that AudioSession is designed to act as the app's central audio session manager. Since the audio session is a process-wide resource on iOS, many apps already have their own manager (mine included), making it difficult to coordinate with Readium without taking on an unwanted dependency or fighting for control.

I'd like to propose providing a small public protocol that extracts the coordination surface Readium consumers need:

public protocol AudioSessionManaging: AnyObject {
    /// Starts a new audio session with the given user.
    func start(with user: AudioSessionUser, isPlaying: Bool)

    /// Ends the current audio session.
    func end(for user: AudioSessionUser)

    /// Indicates whether the user is playing.
    func user(_ user: AudioSessionUser, didChangePlaying isPlaying: Bool)
}

The existing AudioSession class would conform to this protocol and could remain the default value at the relevant injection point (e.g. PublicationSpeechSynthesizer or the top-level navigator), so there would be no breaking changes for existing integrators. The benefit is that external developers who already manage their app's audio session can inject their own implementation, giving them full ownership over activation, deactivation, and interruption handling — which Apple expects to be app-controlled anyway. It also makes the audio layer independently testable.

I would be happy to put together a draft PR if this direction sounds good.

[mickael-menu]

This pain point was raised in another channel, and I originally planned to address it within the upcoming Guided Navigator (which supports media overlays). However, since that won't ship until v4, please feel free to open a PR from develop now. We can then include this fix in 3.11 before we start releasing 4.0.0-alpha.1. Thanks!

[mickael-menu]

Just make sure that this is entirely backward-compatible for v3 🙏

[svenmeyers89]

Thanks, @mickael-menu , and sorry for the delay. Here's the PR with a minimal set of changes to support external audio session handling:
#856