Need a way annotate a property of a schema to say it can only contain extensions of a certain type.

## What are the schemas that are affected by the issue
`experienceevent` with `xdm:endUserIds` needing to only contain properties that ref `\context\identity.schama.json`

## What are examples of products that are impacted by the issue
Adobe Cloud Platform - Schema Registry

Start by examining the `experienceevent` schema and its `xdm:endUserIds` property, then compare the referenced `\context\identity.schama.json` schema. Done means the schema can express and enforce that the property contains only extensions of the specified type in Adobe Cloud Platform Schema Registry.

Written by the indexing model from the issue text.