Added the ability to create external events for the event log

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Starting from V6/V6Preview4, the ability to create external events for the event log has been added. The following methods have been introduced:

IEditSession.CreateJournalEvent() - creates an external event with the specified event source (sender), severity (severity), and event type (eventType). Optionally, the event date (dateTime) can be specified.
IEditSession.SetJournalEventAttribute() - sets attributes for the event. The plugin can use standard attributes for display in the event log or create custom attributes that will be accessible through the Syrve.biz API (/api/0/events/events).
IEditSession.AttachToJournalEvent() - assigns standard order attributes to the event. If the order is a delivery order, it also assigns standard delivery attributes in addition to the order attributes.