Make Time, RealTimeTrip, and StopTimeUpdate interface properties optional

[magnusburton]

It's me again!
My feeds are not compatible with the current types so I wanted to make this update.

When the spec specified a field as Conditionally required I made the property optional. Below is an excerpt from a TripUpdate feed from Sweden.

Feed

entity {
  id: "14010516182011315"
  trip_update {
    trip {
      trip_id: "14010000660994806"
      start_date: "20240521"
      schedule_relationship: SCHEDULED
    }
    stop_time_update {
      stop_sequence: 34
      arrival {
        delay: 0
        time: 1716317220
      }
      departure {
        delay: 0
        time: 1716317220
      }
      stop_id: "9022001000306001"
    }
    stop_time_update {
      stop_sequence: 35
      arrival {
        delay: 0
        time: 1716317400
      }
      departure {
        delay: 0
        time: 1716317400
      }
      stop_id: "9022001000301001"
    }
    vehicle {
      id: "9031008000500547"
    }
    timestamp: 1716316188
  }
}
entity {
  id: "14050001704887972"
  trip_update {
    trip {
      start_time: "18:27:22"
      start_date: "20240521"
      schedule_relationship: CANCELED
      route_id: "9011001001700000"
      direction_id: 1
    }
    stop_time_update {
      stop_sequence: 1
      arrival {
        delay: 0
        time: 1716308842
      }
      departure {
        delay: 0
        time: 1716308842
      }
      stop_id: "9022001001951002"
    }
    stop_time_update {
      stop_sequence: 2
      arrival {
        delay: 0
        time: 1716308842
        uncertainty: 0
      }
      departure {
        delay: 0
        time: 1716308842
      }
      stop_id: "9022001001951001"
    }
    stop_time_update {
      stop_sequence: 3
      arrival {
        delay: 0
        time: 1716308842
      }
      departure {
        delay: 0
        time: 1716308842
      }
      stop_id: "9022001001941001"
    }
    stop_time_update {
      stop_sequence: 4
      arrival {
        delay: 0
        time: 1716308842
      }
      departure {
        delay: 0
        time: 1716308842
      }
      stop_id: "9022001001941002"
    }
    vehicle {
      id: "9031001002500096"
    }
    timestamp: 1716308843
  }
}