Add a "last updated" timestamp to indicate when realtime was last applied on a trip/call

[lassetyr]

Is your feature request related to a problem?
In case of e.g. a realtime-system outage, there is no way for users/apps to find out that the realtime-data may be outdated. Adding a "last updated"-timestamp in the API to indicate when the realtime-data for each trip was last updated would allow apps to indicate this.

Describe the solution you'd like
The "lastUpdated" timestamp should be fetched from the realtime-data itself.

• In SIRI ET EstimatedVehicleJourney this is provided in the field RecordedAtTime
• In a GTFS-RT TripUpdate it looks like timestamp provides a similar value