As a developer I would like to manipulate values of fields property in the contentCreateStruct structure

Implemented getter and setter methods for the fields property of contenCreateStruct structure.
By making these changes I'm able to create a new content and update its fields using JS Rest Client.
https://jira.ez.no/browse/EZP-23276
actually, I would call it `addField` since it only pushes new field without any further checks.