Need an identifier for ExprienceEvents to indicated they represent the same original event observation

## What are the schemas that are affected by the issue
context/experienceevent

## What are examples of products that are impacted by the issue
Analytics, Launch

Need an identifier for ExprienceEvents to indicated they represent the same original event observation. For example a purchase event would cause several events to be raised in different systems that each represent this purchase, but may contain unique information generated by the different systems, where we have the opportunity to add a unique event id right at the event observation it can be included. When these events enter an XDM processing system they would be represented as ExperienceEvents and it would be very useful to have a standard place where this unique identifier be expressed. This would then inform a merging processor that creates a full representation of the purchase from all systems.

Note this is different to the context/experienceevent.@id field which is a way of identifying an instance of an experienceevent for future restating, lets say when further data enrichment is available.

Start with the context/experienceevent schema and compare its existing @id semantics with the proposed identifier for the original event observation. Review how Analytics and Launch represent related purchase events. Done means the XDM model has an agreed standard location and meaning for this identifier, distinct from @id.

Written by the indexing model from the issue text.