Support oneOf in schema definition

[Szer]

oneOf basically means union type
We could encode them with F# DUs

    paths:
      /pets:
        patch:
          requestBody:
            content:
              application/json:
                schema:
                  oneOf:
                    - $ref: '#/components/schemas/Cat'
                    - $ref: '#/components/schemas/Dog'
          responses:
            '200':
              description: Updated
    components:
      schemas:
        Dog:
          ...
        Cat:
          ...

should generate

// usual generation of type objects
type Dog = ... 
type Cat = ... 

type CatOrDog = //generatedName
  | Cat of Cat
  | Dog of Dog