replace the 'start' argument by a 'next' which is now expected to be an (not
so) opaque token used for pagination.
The 'end' argument is dropped.
Adapt tests and the list_origins_by_producer() helper function (in cli.py)
accordingly.
Note: this uses 'next' as opaque token argument name, but this is still to be decided, so I will not lanf it before we agree on a name there.