Improve handling of oneOf

Created by: jmini

With OAS3 it is possible to use oneOf. https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.1.md#schema-object

See one example in composed-oneof.yaml from the test suite.

We should discuss how we want to handle this.

In my opinion (for java) the Schema containing only oneOf entries should be an interface, and all model classes corresponding to the schema mentioned in the oneOf should implement this interface.