Add webhook v3 types

[icholy]

I originally had this function too, but I didn't find it very useful.

// ParseEventData parses the event data into the correct type.
func ParseEventData(e Event) (any, error) {
    var data any
    switch e.EventType {
    case "incident.acknowledged", "incident.annotated", "incident.delegated",
        "incident.escalated", "incident.priority_updated", "incident.reassigned",
        "incident.reopened", "incident.triggered", "incident.unacknowledged":
        data = &IncidentData{}
    case "incident.conference_bridge.updated":
        data = &IncidentConferenceBridgeData{}
    case "incident.custom_field_values.updated":
        data = &IncidentFieldValuesData{}
    case "incident.responder.added", "incident.responder.replied":
        data = &IncidentResponderData{}
    case "incident.status_update_published":
        data = &IncidentStatusUpdateData{}
    case "incident.workflow.started", "incident.workflow.completed":
        data = &IncidentWorkflowInstanceData{}
    case "pagey.ping":
        data = &PageyData{}
    default:
        return nil, fmt.Errorf("unrecognized event type: %q", e.EventType)
    }
    if err := json.Unmarshal(e.Data, data); err != nil {
        return nil, err
    }
    return data, nil
}