Add API method to get indexed object class solr fields

[chalkos]

To better support fieldsToReturn, there should be a way to list all the solr fields for an indexed object class.

[chalkos]

Dynamic fields were obtained using a Luke request which has been discontinued in solr 7.

Maybe this feature could be implemented by returning the schema fields and using a list from configuration to return some extra fields (like the dynamic fields used in ingest crosswalks).