Implement functionality allowing users to specify a custom default playback speed value.

Add functionality allowing users to specify custom default playback speed values instead of being limited to predefined options.

This closes #1546

Changes:

• New: PlaybackSpeedCustomHelpers.swift - Custom speed management and input handling
• Modified: ActionSheetSpecifier.swift - Added custom speed selection logic
• Modified: PlaybackSpeedView.swift - Add logic to retrieve the effective speed value
• Modified: NSObject+SettingsReader.swift - Add handling for "custom" choice, reading custom speed from UserDefaults
• Modified: VLCPlaybackService.m - Apply the actual speed value associated with "playback-speed-custom" key

Known Issue:

1. iOS 17+ shows warning with text input (functionality unaffected):

   This warning appears starting from iOS 17 but does not exist in iOS 16 and below.

   -[RTIInputSystemClient remoteTextInputSessionWithID:performInputOperation:] perform input operation requires a valid sessionID. inputModality = Keyboard, inputOperation = , customInfoType = UIEmojiSearchOperations

2. Warning with the Input Alert Window (functionality unaffected):

   Changing the translatesAutoresizingMaskIntoConstraints property of a UICollectionViewCell that is managed by a UICollectionView is not supported, and will result in incorrect self-sizing. View: <_UIAlertControllerTextFieldViewCollectionCell: 0x125787c10; frame = (0 0; 270 24); gestureRecognizers = <NSArray: 0x60000004c0c0>; layer = <CALayer: 0x6000003739c0>>