Download model versions before starting tasks

https://redmine.teklia.com/issues/11417

The arkindex.ponos.tasks.download_extra_files function should be renamed to download_model_version. It should now download the model version of the task's WorkerRun through task.worker_run.model_version.s3_url, and extract the zstandard archive, into the extra_files/model path just as it does now.