[iOS] Exposing apple pencil and smart highlighter for custom toolbar

[darrenchann]

Add support for apple pencil and smart highlighter in custom toolbar as calling this wouldn't show the tools in this dart toolbar:

var customToolbar = CustomToolbar(
    'myToolbar',
    'myToolbar',
    [
      Tools.annotationLasso,
      Tools.annotationSmartPen,
      Tools.pencilKitDrawing,
      Tools.annotationCreateFreeHand,
      Tools.annotationCreateFreeHighlighter,
      Tools.eraser,
      Tools.annotationCreateSignature,
      Tools.annotationCreateStamp,
    ],
    ToolbarIcons.favorite);
var annotationToolbars = [
  customToolbar,
];
    config.annotationToolbars = annotationToolbars;

[dluco]

Works as expected! 👍