Allow a template provider to define if it should use nested pages hierarchy as a template

Template providers have an interesting option when being used with nested spaces, documented here : https://extensions.xwiki.org/xwiki/bin/view/Extension/Administration%20Application#HUsingahierarchyofNestedPagesastemplate

When creating a template provider that points to a non-terminal page, the expected behavior of XWiki is to actually copy the non-terminal template and its tree of subpages when the template provider is used.

It's an interesting feature, but the fact that this is not configurable raises some limitations for application developers. One example is the following : consider an app that allows users to create sturctures as :
PageA.WebHome <- A page of the application
PageA.TemplateProvider <- A template provider hidden to the user, automatically generated by the application in order to create new documents using PageA.WebHome as a template.

In that case, this design could not work, as there is no way to tell XWiki that in this specific case, the subtree of PageA should *not* be copied over when a new page is created using PageA.WebHome as a template.