Add event types into `upsertor.execute()` method

[mejroslav]

In SeaCat Auth, we would like to add event types when calling upsertor.execute(). Right now, this can be done by adding custom_data = {} argument. But since the key is always the same ("event_type"), we could add into MongoDBUpsertor.execute() method an optional argument event_type which is None by default and can add "event type": event_type in webhook .
See TeskaLabs/seacat-auth#172 (comment)