Add details about `most_recent_task` in the `GET /tasks` endpoint

[benoit74]

Currently the GET /tasks endpoint (used to list tasks) is not returning the most_recent_task of the associated schedule.

This presents the drawback that when we need to display last task in the UI for a bunch of tasks, we have to perform one request per schedule to the GET /schedules/{schedule_name} endpoint. This is causing issues like #882.

As discussed in https://github.com/kiwix/k8s/issues/149#issuecomment-1851442182, should we change this now that things have matured / changed a bit?

[rgaudin]

I'm in favor