Add `started_at` field to cached response documents
Will close https://github.com/huggingface/dataset-viewer/issues/2892

To pass `started_at` field from Job to CachedResponse, I updated `JobInfo` class so that it stores `started_at` info too. It is None by default (and set to actual time by `Queue._start_newest_job_and_delete_others()`). Maybe there is better way to pass this information? 
I think alternatively I can get a new timestamp inside worker's `Loop.process_next_job` and pass it to `finish` to avoid changing dtos, but I don't know if it's better.

Add `started_at` field to cached response documents: Will close https://github.com/huggingface/dataset-viewer/issues/2892

To pass `started_at` field from Job to CachedResponse, I updated `JobInfo` class so that it stores `started_at` info too. It is None by default (and set to actual time by `Queue._start_newest_job_and_delete_others()`). Maybe there is better way to pass this information? 
I think alternatively I can get a new timestamp inside worker's `Loop.process_next_job` and pass it to `finish` to avoid changing dtos, but I don't know if it's better.
