Currently we have two different implementations of mergeJson

The one being used in RestWorkItemHandler
https://github.com/kiegroup/kogito-runtimes/blob/9bd1c5ccc3c5d297e44d2001286de0befd81e0d7/kogito-workitems/kogito-rest-workitem/src/main/java/org/kogito/workitem/rest/RestWorkItemHandlerUtils.java

The one being used in OpenAPIHandler
https://github.com/kiegroup/kogito-runtimes/blob/master/kogito-workitems/kogito-openapi-workitem/src/main/java/org/kogito/workitem/openapi/JsonNodeResultHandler.java#L41

We should move one of the two to a common static class and invoke it from both handlers.
In order to chose one of the two, I think we need some performance metrics