on<T> method
Register the typed handler for one event. The payload is cast to T.
Use the entity type for entity events (UserDto), Mutation<UserDto>
for user.updated, List for batch events, etc.
Implementation
void on<T>(String event, NorbixWebhookHandler<T> handler) {
_handlers[event] = (payload, e) => handler(payload as T, e);
}