Fix iOSBroadcastExtension always false after copyWith invoked

[kNoAPP]

Expected
When I pass true to useiOSBroadcastExtension, the screen-share video track should invoke getDisplayMedia with constraints containing deviceId set to broadcast.

Actual
When I pass true to useiOSBroadcastExtension, the screen-share video instead leaves constraint deviceId unset because a prior call to ScreenShareCaptureOptions#copyWith always sets useiOSBroadcastExtension to the default value of false.

  /// Creates a LocalTracks(audio/video) from the display.
  ///
  /// The current API is mainly used to capture audio when chrome captures tab,
  /// but in the future it can also be used for flutter native to open audio
  /// capture device when capturing screen
  static Future<List<LocalTrack>> createScreenShareTracksWithAudio([
    ScreenShareCaptureOptions? options,
  ]) async {
    if (options == null) {
      options = const ScreenShareCaptureOptions(captureScreenAudio: true);
    } else {
      options = options.copyWith(captureScreenAudio: true); // <-- This line produces the bug
    }

[...]

Fix
Added support to ScreenShareCaptureOptions#copyWith to preserve or overwrite the value of useiOSBroadcastExtension.

[cloudwebrtc]

lgtm