Fix timings begin_date and end_date JSON export

[jcamiel]

With --json option, timings are serialized to JSON like this:

          "timings": {
            "app_connect": 0,
            "begin_call": "2024-04-18 14:09:00.513250 UTC",
            "connect": 2412,
            "end_call": "2024-04-18 14:09:00.521567167 UTC",
            "name_lookup": 31,
            "pre_transfer": 2459,
            "start_transfer": 8035,
            "total": 8185
          }

begin_call and end_callshould be serialized to string using RFC-3339, with a microseconds maximum precision (nanoseconds are not supported in Python for instance):

          "timings": {
            "app_connect": 0,
            "begin_call": "2024-04-18T15:48:53.981976Z",
            "connect": 2376,
            "end_call": "2024-04-18T15:48:53.990433Z",
            "name_lookup": 23,
            "pre_transfer": 2418,
            "start_transfer": 8312,
            "total": 8389
          }