[Bug] JavaScript client generator should serialize/deserialize polymorphic types

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• [x ] Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

The generated ApiClient.js does not add the discriminator value to the output when it serializes a model. This prevents the server from processing requests where the concrete type is not known ahead of time.

Conversely, the generated ApiClient.js does not make use of the discriminator value to construct the correct concrete type when it deserializes a response.

openapi-generator version

4.3.2

OpenAPI declaration file content or url

components:
  schemas:
    Node:
      type: 'object'
      properties:
        items:
          type: 'array'
          items:
            $ref: '#/components/schemas/NodeItem'
    NodeItem:
      type: 'object'
      discriminator:
        propertyName: 'type'
        mapping:
          ConcreteItem: '#/components/schemas/ConcreteItem'   
        properties:
          id:
            type: 'string'
    ConcreteItem:
      allOf:
        - $ref: '#/components/schemas/NodeItem'
        - type: 'object'
          properties:
            prop1:
              type: 'string'

Generation Details

Steps to reproduce

Related issues/PRs

Suggest a fix

The way I currently work around this is by editing the generated ApiClient.js and patching both callApi and convertToType.

Ideally, the generator would generate a ModelMapper, plug that directly into superagent, and allow the end user to replace it if they would like.

Start with the provided OpenAPI YAML and the generated ApiClient.js, focusing on the callApi and convertToType paths mentioned in the report. Reproduce generation with the Node/ConcreteItem discriminator example; done means serialization includes the discriminator value and deserialization constructs the mapped concrete type.