[typescript-angular] failed to generate tagged unions

Description

Generating typescript-angular code with taggedUnions enabled throws an Exception:

Exception in thread "Thread-1" java.lang.RuntimeException: Could not generate model 'BillingSectionConfig'
        at io.swagger.codegen.v3.DefaultGenerator.generateModels(DefaultGenerator.java:451)
        at io.swagger.codegen.v3.DefaultGenerator.generate(DefaultGenerator.java:779)
        at io.swagger.codegen.v3.cli.cmd.Generate.run(Generate.java:378)
        at java.lang.Thread.run(Thread.java:748)
Caused by: com.github.jknack.handlebars.HandlebarsException: /handlebars/typescript-angular/modelGenericEnums.mustache:1:3: java.lang.ClassCastException: java.lang.String cannot be cast to io.swagger.codegen.v3.VendorExtendable
    /handlebars/typescript-angular/modelGenericEnums.mustache:1:3
        at io.swagger.codegen.v3.generators.handlebars.ExtensionHelper.apply(ExtensionHelper.java:11)
...

Swagger-codegen version

<dependency>
  <groupId>io.swagger.codegen.v3</groupId>
  <artifactId>swagger-codegen-cli</artifactId>
  <version>3.0.22</version>
</dependency>

Swagger declaration file content or url

The relevant parts that would create tagged union interfaces. This was generated by

<plugin>
  <groupId>io.swagger.core.v3</groupId>
  <artifactId>swagger-maven-plugin</artifactId>
  <version>2.1.5</version>
</plugin>

openapi: 3.0.1
...
components:
  schemas:
    BillingSectionConfig:
      type: object
      allOf:
      - $ref: '#/components/schemas/RequestSectionConfig'
      - $ref: '#/components/schemas/RequestSectionConfigs'
    ContactSectionConfig:
      type: object
      allOf:
      - $ref: '#/components/schemas/RequestSectionConfig'
      - $ref: '#/components/schemas/RequestSectionConfigs'
    RequestSectionConfig:
      type: object
      properties:
        displayName:
          type: string
        description:
          type: string
    RequestSectionConfigs:
      required:
      - type
      type: object
      properties:
        type:
          type: string
      discriminator:
        propertyName: type
        mapping:
          BillingSectionConfig: '#/components/schemas/BillingSectionConfig'
          ContactSectionConfig: '#/components/schemas/ContactSectionConfig'
      oneOf:
      - $ref: '#/components/schemas/BillingSectionConfig'
      - $ref: '#/components/schemas/ContactSectionConfig'

Command line used for generation

java -jar swagger-codegen-cli-3.0.22.jar generate -l typescript-angular -i ../rat-rest/target/swagger/jaxrs-api.yaml -o src/app/api --additional-properties="taggedUnions=true"

Steps to reproduce

Run the command used for generation.

Related issues/PRs

PR: #7245

Suggest a fix/enhancement

I guess a few changes are necessary to get this working:

1. In io.swagger.codegen.v3.CodegenProperty the missing getter/setter for discriminatorValue needs to be added. Otherwise the template engine will ignore this property

2. In the swagger-codegen-generators project the discriminatorValue is not calculated correctly. Currently the children of a CodegenModel are used but it should be the mapping of the discriminator

3. Also in the swagger-codegen-generators project: The modelTaggedUnion.mustache does not support inheritance and does not generate the discrimator type correctly

I guess that I am able to provide PR(s) fixing these issues with some guidance. Please let me know if you would accept one

Reproduce the failure with the provided typescript-angular command and taggedUnions=true, then inspect CodegenProperty, the discriminator handling in swagger-codegen-generators, and modelGenericEnums.mustache/modelTaggedUnion.mustache. Done means generation completes for the supplied schemas and produces tagged-union interfaces with the discriminator values and inheritance represented correctly.