Registers a new webhook to receive event notifications. Subscribe to one or more event types
via notificationTypes:

• FNOL (Fnol = 1) — insurance-claim lifecycle for a detected accident.
  Delivered at three notification levels (1 = first detection, 2 = verified, 3 = report-ready).
  Payload fields: EventId, EventTime, Vin, Connection, TSP, Location, NotificationLevel, ReportReady, IsAccurate.
• Safety Event (EventNotification = 2) — a detected driving safety event (e.g. harsh braking, collision)
  with an Event Risk rating from 1 (very low) to 5 (critical).
  Payload fields: EventId, IssuedOn, TranslatedIncidentTypes, EventRisk, VehicleId, VehicleName, VehicleVin, VehicleGroups, Recipients.

The webhook receives a POST or PUT request with the envelope:
{ "timestamp": "...", "type": <ManifoldNotificationType>, "data": { ... } }
where data contains the payload matching the event type.