Improve generation of object composition/inheritance on properties

Description

When a model is generated using swagger-codegen for openapi 3.0, if the specification contains inheritance/composition on a property, the generated model does not link the different classes properly.

This behaviour has been observed in the Kotlin and Swift4 generated models, but probably affects all the languages.

Swagger-codegen version

swagger-codegen 3.0.7

Swagger declaration file content or url

components:
  schemas:
    BaseModel:
      type: object
      properties:
        title:
          type: string
    ExtendedModel:
      properties:
        attributes:
          allOf:
            - $ref: '#/components/schemas/BaseModel'
            - type: object

The above specification produces these two classes:

public struct BaseModel: Codable {
    public var title: String?
    public init(title: String?) { 
        self.title = title
    }
}

public struct ExtendedModel: Codable {
    public var attributes: Object?
    public init(attributes: Object?) { 
        self.attributes = attributes
    }
}

As this example shows, the ExtendedModel class contains a property of type Object (that by the way does not exist in Swift) instead of pointing to the BaseModel class defined before.

Start by reproducing the OpenAPI 3.0 schema shown in the issue with the swagger-codegen 3.0.7 generation entry point, focusing on the Kotlin and Swift4 outputs. Trace how an allOf property reference is resolved and compare the generated ExtendedModel property with the referenced BaseModel. Done means the property uses the appropriate generated model instead of Object, with behavior verified for the reported generators.