DMN complementary REST endpoints

Motivation: the current available DMN REST endpoints are not enough to cover all the use-cases which are known from previous experience.
Goals: provide DMN endpoints that cover the basic use case (introspect, evaluate, evaluate technically) for DMN.
Impact: additional codegenerated REST endpoint will be present.

A complementary set of REST endpoints for DMN on Kogito:

• evaluate model and return generic DMNResult
• GET model as XML specification but without decision-logic (for model introspection using the standard format)

As discussed at the time with Edson, and as described in the circulated design document.