[BUG] Nested containers in response not supported

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?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

Issue is because the CodegenResponse has no way of specifying the nested type when the response from the operation is being parsed. This means mustache templates cannot have a way to detect that the response is actually nested container because the item type of the root container are not currently specified.

openapi-generator version

4.0.0

OpenAPI declaration file content or url

  '/products/names':
    get:
      tags:
        - Debug
      summary: Retrieves list of names 
      description: ''
      operationId: getProductNames
      produces:
        - application/json
      responses:
        '200':
          description: successful operation
          schema:
            type: array
            items:
              type: array
              items:
                type: object
        '400':
          description: not initialized
        '500':
          description: Internal error

Suggest a fix

CodegenResponse should have additional fields or extension of current fields indicating the items of the container when the flag isContainer is set.

Start with CodegenResponse and the response-parsing path described in the issue, then inspect the mustache templates that consume container metadata. Reproduce the nested array response from the supplied OpenAPI YAML and compare the generated output; done means templates can distinguish the nested container and its item type.