`oneOf` support via Discriminated Unions

[rrichardson]

Many of the target languages support Union types.
e.g. for TypeScript, I think oneOf could be rendered as Discriminated Unions.

Does openapi-codegen support these at the moment? I see one mention of it in the templates, inside of the htmlDocs2 template. It looks like it is just introspecting the root object, though. It would be nice to see them treated as a 1st class template object similar to enums and "generics"

Example

components:
  schemas:
    ObjA:
      properties: ...
    ObjB:
      properties: ...
    ObjC:
      properties: ...
    Foo:
      oneOf:
        - $ref: "#/components/schemas/ObjA"
        - $ref: "#/components/schemas/ObjB"
        - $ref: "#/components/schemas/ObjC"

Should result in

export type Foo =  ObjA | ObjB | ObjC;

[MikeRalphson]

It would be helpful if you could give an example of what needs providing in the model to be able to support this in the templates?