Add duration to cached steps
Will close https://github.com/huggingface/dataset-viewer/issues/2892

As suggested in https://github.com/huggingface/dataset-viewer/pull/2908#pullrequestreview-2126425919, this adds duration field to cached responses. Duration is computed using `started_at` field of a corresponding job.

Note that this PR adds new fields to major libcommon dtos `JobInfo` and `JobResult`. 
Add duration to cached steps: Will close https://github.com/huggingface/dataset-viewer/issues/2892

As suggested in https://github.com/huggingface/dataset-viewer/pull/2908#pullrequestreview-2126425919, this adds duration field to cached responses. Duration is computed using `started_at` field of a corresponding job.

Note that this PR adds new fields to major libcommon dtos `JobInfo` and `JobResult`. 