Refactor AudioPlayer to use an AudioPlayerRateManager protocol for managing rate/defaultRate state management. Implements a LegacyAudioPlayerRateManager implementation with existing (pre-iOS 16) behavior.

ios/App/Shared/player/AudioPlayerRateManager.swift

@@ -0,0 +1,23 @@
+//
+//  AudioPlayerRateManager.swift
+//  Audiobookshelf
+//
+//  Created by Marke Hallowell on 4/14/24.
+//
+
+import Foundation
+import AVFoundation
+
+protocol AudioPlayerRateManager {
+    var rate: Float { get }
+    var defaultRate: Float { get }
+    var rateChangedCompletion: () -> Void { get set }
+    var defaultRateChangedCompletion: () -> Void { get set }
+    
+    init(audioPlayer: AVPlayer, defaultRate: Float)
+    
+    func setPlaybackRate(_ rate: Float)
+
+    // Callback for play events (e.g. LegacyAudioPlayerRateManager uses this set rate immediately after playback resumes)
+    func handlePlayEvent() -> Void
+}