extensions specifying a version for namespace schema

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In the namespaces declaration of an extension, within schema, you have the ability to reference another namespace, however you cannot specify the version of the namespace you are referencing:

https://schema-language.readthedocs.io/en/latest/description.html#schema

This could cause a problem if an extension extends from core or from another extension and then backward-incompatible changes are made to the extension being referenced.