DMN demo extended builtin function for runtime

Demonstrate how the developer/user can create a custom FEEL function to access to the DMNRuntime.

This in a target application can serve several use cases:

• Introspection. For instance return information about the environment, like number of models etc.
• Access to Kie API. For instance to call out to a Stateless Kie Session execution in the same application. (The DMN user is knowledgeable by the spec external invocation shall be side-effect free