Implement support for optional objects in openapi 3.1.0

[nils-degroot]

Currently mapping from openapi 3.1.0 nullable types only works with the types array which does not support object. This pr provides mapping for oneOf (object | null) to the legacy style mapping. See the following input openapi spec:

openapi: 3.1.0
components:
  schemas:
    NullableObject:
      type: object
      properties:
        field:
          oneOf:
            - type: 'null'
            - $ref: '#/components/schemas/TheObject'
    TheObject:
      properties:
        name:
          type: string

It would be updated to this openapi spec after this pr:

openapi: 3.1.0
components:
  schemas:
    NullableObject:
      type: object
      properties:
        field:
          allOf:
            - $ref: '#/components/schemas/TheObject'
          nullable: true
    TheObject:
      properties:
        name:
          type: string

[cjbooms]

Nice one. Thanks for the contribution