Define a structure for Semantic Linking

Semantic Linking is a metadata field of the Output Port that contains all the fields of the schema that can be joinable with other Output Ports' fields (even from other Data Products inside other domains).

Right now the Semantic Linking is a generic object, but it should be better defined with a basic structure.
The structure should define a way to reference external Output Ports' fields for some of the output port's fields.