Add extension point for custom node type in document tree macro

Before XWIKI-24216, it was possible to add custom node type, so with the macro #maybeAddNode it was possible to handle custom type. So $nodeType could be custom and then the custom usage just needed to provide a custom macro for #maybeAdd<costom name>Node.

Now it's not possible any more because the type of node is hard coded. So this issue is to introduce an extension point in a way to restore this feature.

Note: XWiki SAS is using this for the procedure Flavor.