Resolve "Implement Lambda: update_vehicles_count"

Closes #26 (closed)

Tested by manual invocation in the browser using the following JSON:

{
  "predictFor": 1704067200,
  "cameraId": "8ca14933-fe9e-4e50-bf89-e05d0e2bf863",
  "counts": {
      "carCountPrediction": 10,
      "emergencyVehicleCount": 0
  }
}