Adding a field in trip objects to cover cancelled trips easier

When integrating the taxi and passenger services data, we realized it was complex today to send a trip object in cases where the trip was cancelled for whatever reason (no-show, app-cancellation, etc...). This is because the only way to determine the trip was cancelled is to go into the mode-specific trip attributes and infer the cancellation from the cancellation_reason field being not-empty (as of today, only an unnormalized string of text).

=> We suggest to add a trip_cancelled boolean as part of the root trip object so that the cancellation can be determined a lot easier. In this case, fields such as distance, duration, etc... could be made optional when trip has been cancelled for whatever reason.

This could be made mode-specific if some other mode do not need such a field.

PS: See issue #888 for our take on the mode-specific attributes and how we suggest to structure them.

Start by reviewing the trip object definition and issue #888, then determine whether cancellation belongs on the root object or in mode-specific attributes. Define how the boolean interacts with optional fields such as distance and duration, and ensure the specification gives one unambiguous way to identify cancelled trips.