Add the publication_time field in the telemetry object

[pierre-bouffort]

Is your feature request related to a problem? Please describe.

We are often using the publication_time field provided for all events to monitor the latency SLAs cities may have on the data flowing in from the operators through MDS. As I read through the specification for the telemetry object, we noticed there isn't such field which seems like a simple fix.

Describe the solution you'd like

We could add the publication_time field as an optional timestamp field in the telemetries.

Is this a breaking change

No, not breaking

Impacted Spec

The spec that would be impacted is the data_type page https://github.com/openmobilityfoundation/mobility-data-specification/blob/main/data-types.md#telemetry