Add hasContent enricher

Add an hasContent enricher server side to tell if a `Folderish` or `Collection` container does have content (boolean).

On Web UI, this is yet another tiny thing to load when it comes to fetch a document but at the same time we could leverage it in our table, grid, list elements when it comes to display a Folder/Collection in Web UI: this could spare us useless requests.
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We must also deprecate children json enricher

ChildrenJsonEnricher.java relies on

wrapper.getSession().getChildren(document.getRef());

which fetches all children -> does not scale.

Better rely on search endpoints with document_content page provider or @children adapter which provides pagination.

+ add a comment in https://doc.nuxeo.com/nxdoc/content-enrichers/