Only output the S3 PUT URL of element thumbnails for the thumbnail generation task

The thumbnail_put_url attribute on elements is only useful to the arkindex_tasks.generate_thumbnails task. We should only return this URL when element endpoints are called with Ponos task authentication for this specific task.

Restricting to the process can be done by checking whether thumbnail generation is enabled on the process. We can further restrict on the specific task (to prevent rogue workers from accessing this URL) by checking whether the tasks' Docker image is settings.ARKINDEX_TASKS_IMAGE and whether the command runs arkindex_tasks.generate_thumbnails.