Need mechanism to link multiple Experience Events that logically correspond to a single action

This issue was raised in review of PR #509. Adobe solutions like Target currently use an extended field called a "supplementary data ID" to link events posted from different components that make up a single logical action or experience. We should probably have this capability as part of the core definition of ExperienceEvent since it is likely a common pattern.

## What are the schemas that are affected by the issue
ExperienceEvent and extensions

## What are examples of products that are impacted by the issue
Analytics, Target, Ad Cloud

Start by reviewing PR #509 and the current ExperienceEvent and extension schemas. Determine how a core mechanism could link events from different components into one logical action, then confirm the resulting definition covers the Analytics, Target, and Ad Cloud examples described here.

Written by the indexing model from the issue text.