Update Editor to support factory pattern/polymorphism as employed by OpenAPI standard

[kevinkirkl3y]

We want to be able to use the oneOf, anyOf keywords in addition to the allOf keyword currently employed by the editor.

This will require changes to the UI: The editor with default to the use of allOf when defining objects. Then add a dropdown menu that will allow users to select oneOf or allOf with the default being allOf. Selection of the oneOf keyword would then activate an additional dropdown requiring the user to select the Item Type.

Swagger/OpenAPI documentation:
https://swagger.io/docs/specification/data-models/oneof-anyof-allof-not/
https://swagger.io/docs/specification/data-models/

[cwhanse]

To add oneOf capability that implements a factory pattern, the editor needs to have capability to create/edit/delete a response object, with a discriminator attribute.