[Python] Multilevel inheritance discriminator support

Created by: eak24

Bug Report Checklist

Have you provided a full/minimal spec to reproduce the issue?
Have you validated the input using an OpenAPI validator (example)?
What's the version of OpenAPI Generator used? 4.2.3
Have you search for related issues/PRs? Yes - they are #4446
What's the actual output vs expected output?
[Optional] Bounty to sponsor the fix (example)

Description

@spacether

When using OAS to describe a complex polymorphic graph, the client is currently unable to serialize grandchildren of any given type (subclasses of subclasses.) I have made a recursive model lookup function, which recursively looks at the discriminators of the subclasses (discriminated classes) until the leaf nodes, but that has very bad performance, reducing performance to 1/100 of normal deserialization. There is a lot of repeated work done for the same model over and over, so I added caching, and the performance is now roughly 1/10 of what it was. Ideally, there would be some way to specify the whole type mapping dictionary.

openapi-generator version

4.2.3

OpenAPI declaration file content or url

components:
  schemas:
    Animal:
      type: object
      required:
      - type_key
      properties:
        type_key:
          type: string
      mapping:
        pet: '#/components/schemas/Pet'
    Pet:
      type: object
      properties:
        type_key:
          type: string
      discriminator:
        propertyName: type_key
        mapping:
          cat: '#/components/schemas/Cat'
    Cat:
      type: object
      properties:
        type_key:
          age: number

Suggest a fix

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