EZP-23276: Added a method to manipulate the fields in the contentCreateStruct

JIRA: https://jira.ez.no/browse/EZP-23276

# Description

This patch adds a method to easily add a field value in the content create struct. This PR replaces #45.

# Tests

unit tests + manual tests in https://github.com/ezsystems/PlatformUIBundle/pull/113