[BUG] [angular-typescript] Unused models not generated

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

When defining model entities that are not used directly by the resscources these models are not generated with the typescript-angular generator. (see SpecialPet in the example below).
This only happens when the model is in a separate file!

This is a problem since our api uses the allOf to extend a base class. The ressource formally returns the base class, but depending on inputs, different acutal responses (subclasses) are returned. It is clear that there needs to be some manual casting, but without the subclass at hand we'd need to manually code it.

openapi-generator version

4.0.2

OpenAPI declaration file content or url

api.yaml:

openapi: 3.0.0
info:
  version: "1.0.0"
  title: "Swagger Petstore"

paths:
  /pet/findByStatus:
    get:
      responses:
        200:
          description: "successful operation"
          content:
            application/json:
              schema:
                type: "array"
                items:
                  $ref: 'types.yaml#/components/schemas/Pet'
        400:
          description: "Invalid status value"

types.yaml:

openapi: 3.0.0
info:
  version: "1.0.0"
  title: "Swagger Petstore"

paths:
  /dummy:
    get:
      responses:
        200:
          description: 'dummy'
  
components:
  schemas:
    Pet:
      type: "object"
      properties:
        id:
          type: "integer"
          format: "int64"
        name:
          type: "string"
          example: "doggie"
    SpecialPet:
      type: "object"
      allOf: 
      - $ref: '#/components/schemas/Pet'
      - type: object
        properties:
          specialType:
            type: string

Command line used for generation

java -jar openapi-generator-cli-4.0.2.jar generate -i api.yaml -g typescript-angular

Steps to reproduce

Related issues/PRs

Suggest a fix

There should be an option to allow generation of unused models.

Alternatively/Additionally secondary input files from which models are generated might as well solve the problem.

Reproduce the issue with api.yaml and types.yaml using the listed OpenAPI Generator 4.0.2 command for typescript-angular. Check why the separately defined SpecialPet model is omitted when it is not referenced directly by a resource. Done means unused models such as SpecialPet are generated, ideally with an option or behavior that covers external model files.