Add API to get all descendants of a page, based on the reference hierarchy

Currently there is an API called getChildren() and getChildrenReferences() in com.xpn.xwiki.api.Document , and they both fetch children of the document based on the old parent child relation (not on the page references, the "nested spaces" hierarchy) - see more here https://www.xwiki.org/xwiki/bin/view/Documentation/UserGuide/Features/ContentOrganization/NestedPagesMigration/#HHistory .

There should be an API, probably in com.xpn.xwiki.api.Document as well, allowing to fetch descendants of a page based on the hierarchy, potentially with various parameters / configuration: only direct children, all descendants, etc.

The only alternative today is making a query in the database, after having understood the database model of XWiki.