Store model_version on WorkerRun during a TrainingProcess

The ponos agent automatically downloads any object (and model) set as extra_files, that's how generic workers work. The frontend updates the worker run's model_version attribute to set the model's download URL.

Now that we want to implement fine-tuning on Arkindex, we need to be able to do the same during training processes. We just need to pass the selected model_version here when creating the WorkerRuns and the tasks should be built correctly.