Lookup DimensionSpec - Change Map

[mbram13]

Not sure if this is the correct repo for the issue I'm reporting.

The lookup dimension spec (marked as experimental) makes marshaling models extremely difficult in Scala/Java (I use case classes to build our adapter models). The inner map element/object requires a dynamically generated key property. Recommend adding/replacing current approach with {"key": "my-key", "value": "my-value"}