Add enrichers argument to Documents.get_children()

Issue

For NXDRIVE-1838 to be efficient, it would be very useful to be able to fetch permissions of documents when retrieving them.

Currently, to get all children of a given parent folder, this code works well:

docs = server.documents.get_children(path="/")

But for NXDRIVE-1838, it is needed to know if the user has write access to all folderish documents. And so another call will be made for each folder:

# Peuso-code
can_write = server.documents.has_permission(doc.uid, "ReadWrite")

This would explode any decent server.

Proposition

Add a new argument to Documents.API.get_children() to be able to fetch more than basic information:

def get_children(self, uid=None, path=None, enrichers=None):
    # type: (Optional[Text], Optional[Text], Optional[List[str]]) -> List[Document]
    return super(API, self).get(
        path=self._path(uid=uid, path=path), enrichers=enrichers, adapter="children"
    )

And the new call would be:

docs = server.documents.get_children(path="/", enrichers=["permissions"])