[core] [regression] set parentName when a single possible parent exists

Created by: mnahkies

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Description of the PR

Whilst the spec states that the 'allOf' relationship does not imply a hierarchy:

While composition offers model extensibility, it does not imply a hierarchy between the models. To support polymorphism, the OpenAPI Specification adds the discriminator field.

Unfortunately this does not make sense for many existing use cases, that were supported by older versions of the generator. Therefore, I've restored the older behavior, specifically in the case that only a single possible parent schema is present.

I think a more complete solution would generate interfaces for the composed schemas, and mark the generated class as implementing these.

@wing328 I guess you're probably the best person to review this, given that you overhauled the support for anyOf, allOf, oneOf originally.

I'd be interested if you had an alternative suggestion for achieving this without requiring the parent schema to know the identity of all the extending schema's - in our real definition collection, we have a set of common models that many different entities extend from in other definition files, and we have also build some common library functions that we use on the generated sub classes that accept objects extending from the parent.

Admittedly, our method of using allOf to express this feels awkward. Ideally I'd like a way to generate a generic Page<T> class that could then be used as Page<Cat>, Page<Dog>, etc but I'm not aware of anyway to do this with the openapi spec / code gen tooling.

Example YAML:

openapi: 3.0.0
info:
  title: example
  description: example
  version: 0.0.0
paths:
  /test:
    get:
      tags:
        - Test
      summary: test
      description: test
      operationId: getCats
      responses:
        '200':
          description: successful
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/PagedCats'
components:
  schemas:
    Page:
      properties:
        page:
          type: integer
      required:
        - page
    Cat:
      properties:
        id:
          type: string
          format: uuid
        required:
          - id
    Dog:
      properties:
        id:
          type: string
          format: uuid
        required:
          - id
    PagedCats:
      allOf:
        - $ref: '#/components/schemas/Page'
        - properties:
            items:
              type: array
              items:
                $ref: '#/components/schemas/Cat'
          required:
            - items
    PagedDogs:
      allOf:
        - $ref: '#/components/schemas/Page'
        - properties:
            items:
              type: array
              items:
                $ref: '#/components/schemas/Dog'
          required:
            - items

fixes #2845 (closed), and fixes #3523 (closed)