Walk utility function does not walk lazy child nodes

Hi,

As far as I am aware the documentation on lazy loading is a bit scarce , but some of the nodes `children` on our usage of the tree are an implementation of [GetTreeItemChildrenFn](https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/react-sortable-tree/index.d.ts).

Now we have a problem , we need to programmatically expand the node before we can copy its content to clipboard. I tried to approach this with the [walk](https://github.com/frontend-collective/react-sortable-tree/blob/master/src/utils/tree-data-utils.js) with the `ignoreCollapsed` set to false. However this does not trigger the lazy load to be expanded programmatically , nor does directly changing `node.expanded` field.

Any idea how one could expand a lazy node programmatically? Thanks