Next gen DMN kie-server endpoints

Provide more user-friendly DMN endpoints on KIE Server with better Swagger documentation that will present more accurate inputs/outputs json payloads.

New endpoints:

• GET /server/containers/{containerId}/dmn/openapi.json (|.yaml)
  Retrieves swagger/OAS for the DMN models in the kjar project
• GET /server/containers/{containerId}/dmn/models/{modelname}
  Standard DMN XML but without any decision logic
• POST /server/containers/{containerId}/dmn/models/{modelname}
  json-only evaluation of a specific DMN model with a body payload tailored for the specific model
• POST /server/containers/{containerId}/dmn/models/{modelname}/{decisionServiceName}
  json-only evaluation of a specific decision service of a specific DMN model with a body payload tailored for the specific model
• POST /server/containers/{containerId}/dmn/models/{modelname}/dmnresult
  json-only evaluation of a specific DMN model with a body payload tailored for the specific model, but returning a json representation as a DMNResult
• POST /server/containers/{containerId}/dmn/models/{modelname}/{decisionServiceName}/dmnresult
  json-only evaluation of a specific decision service of a specific DMN model with a body payload tailored for the specific model, but returning a json representation as a DMNResult

The new endpoints will be added to our existing kie-server-rest-dmn extension: https://github.com/kiegroup/droolsjbpm-integration/tree/master/kie-server-parent/kie-server-remote/kie-server-rest/kie-server-rest-dmn

Additional acceptance criteria:

• Resolution on what to do in case of "name collisions"
• The original DMN endpoints will stay intact (should they be deprecated?):
  /server/containers/{containerId}/dmn