Add worker version option to support models

The model dataimport.WorkerVersion requires a new boolean field named model_usage (similar to existing field gpu_usage), by default it's set to False.

The field must be manageable through the admin interface.

It also must be supported in different endpoints:

• readable in ListWorkerVersions
• readable in RetrieveWorkerVersions
• writable in CreateWorkerVersion

Do not update the code in StartProcess, it will be made in a followup