LPS-49633 removed flexjson, so StringTransformer had to be changed; fixed an issue with StringTransformer

Please check the changes. This is _not_ only simple migration to new library, since I think I fixed one issue - when no special case is active, we now return an quoted string, not just raw string. In other words, only the special case is treated differently. That is different from what we had in previous code, but I dont know if this is on purpose or it was an issue.

Once when this gets merge, would you be so kind to update the portal with new jar, so I can add fix there, too. Thank you!