sdfType for links (or core-links)

[cabo]

Over at OneDM, we have been discussing the use of links in SDF:
https://github.com/t2trg/wishi/wiki/NOTE:-Link-information-for-data-models

This clearly calls for an sdfType for links.
With the sdfType registry (#82), we don't need to define this sdfType in the main document.

The sdfType could go with a representation type (type) that is a JSO object, with the fields being target attribute names registered in the new CoRE target attribute registry.

[cabo]

Covered by #82 registry for sdfTypes