Add optional free text message to `CarpoolBookingEvent`

[pierrecamilleri]

The CarpoolBookingEvent should have an optional free text message property, e.g. "message", to allow the carpooling operator to give some additional information about an event (e.g. a booking cancellation).

If I remember well, it has been discussed with the working group, and there was no objection to this, it just has never been implemented.