proper pagination for IndexerStorage.origin_intrinsic_metadata_search_by_producer

replace the 'start' argument by a 'page_token' 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.

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Migrated from D2222 (view on Phabricator)