[V2] References between events or external entities & "event.upsert"

[perki]

To synchronize events from external sources, the best way to do, as of now, was to create a streamId with the external ID to "tag" an event and make a cross reference.
The same logic when associating multiple events, for example, data extracted from a PDF were linked to this PDF by a custom streamId. While functional, this has proven limitations in particular when constructing Access Right Logic with a large number of streams.

Events should propose indexed feeds (mentionned in #513 ) to retrieve associated events.

Proposal add two fields to events
1- external references: externaIds as a key:value object.
2- related events: relatedEvents as an Array of eventId

To facilitate syncronization of updates between two systems events.upsert method should be introduced based on externalIds

To facilited retrieval or related events events.get should implement includesRelatives