[FEATURE] Add test start time to task list API

[dri-mobisec]

It would be helpful to have a "started_at" field in the tasks list JSON.
With this field one could be able to understand if a test has started or not and, once a test is finished, how much time elapsed from start to finish.
The queued_at field alone is obviously not enough because id doesn't allow to know if a test has started or not, you just know that in a certain point in time a test has been queued

[github-actions]

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