Expose dataset properties as pseudo/extra fields about dkan

[dafeder]

Taking a look at this -- in general I'm wary of making promises of integrating with Drupal field system that will be too difficult/complex to keep with different kinds of schemas. But I'll reserve judgement till I've taken your PR for a spin :)

[stefan-korn]

@dafeder : I understand that you are wary of this.

But if one plans to use heavily customized schema (and not planning to use the React frontend) the preprocessing at metastore_preprocess_node__data and the fixed node template seems kind of very hardcoded and lack flexibility imho. Getting the schema properties back in the Drupal fields with using the Search API implementation seems very flexible to me, so I would at least like to share this with you. We can realize a few things with this solution very easy without doing too much hardcoding.

[swirtSJW]

I see a real power in this, especially as it paves the way for being able to surface JSON Metadata elements in Views.