[BUG][DefaultCodegen] allOf not respecting readOnly and all

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?
• 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

I try to use a enum property, but override readOnly attribute (same for description, and all others), but it is not working.
I only noticed, because I'm having my own template script (not related to this problem!) to not generate @NotNull anotation in case of required but readOnly attribut, like here: #9655

This is the proposed way of dealing with readOnly consider by OpenAPI spec / JSON schema, see:

• https://github.com/OAI/OpenAPI-Specification/issues/872#issuecomment-339847815
• https://github.com/OAI/OpenAPI-Specification/issues/1671#issuecomment-414690621

openapi-generator version

latest version, master.

OpenAPI declaration file content or url

File src/test/resources/3_0/allOf-readonly.yaml

openapi: 3.0.1
info:
  version: 1.0.0
  title: Example
  license:
    name: MIT
servers:
  - url: http://api.example.xyz/v1
paths:
  /person/display/{personId}:
    get:
      parameters:
        - name: personId
          in: path
          required: true
          description: The id of the person to retrieve
          schema:
            type: string
      operationId: list
      responses:
        '200':
          description: OK
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/clubForCreation"
components:
  schemas:
    ClubType:
      type: string
      enum:
        - serious
        - silly

    clubForCreation:
      properties:
        type:
          $ref: '#/components/schemas/ClubType'

    clubForUpdate:
      properties:
        type:
          allOf:
            - $ref: '#/components/schemas/ClubType'
            - readOnly: true

Generation Details

Steps to reproduce

Test for DefaultCodegenTest

    public void testAllOfReadonly() {
        final OpenAPI openAPI = TestUtils.parseFlattenSpec("src/test/resources/3_0/allOf-readonly.yaml");
        DefaultCodegen codegen = new DefaultCodegen();

        // OK
        Schema clubForCreation = openAPI.getComponents().getSchemas().get("clubForCreation");//clubForCreation.getProperties().get("type")).getReadOnly() is null
        CodegenProperty clubForCreationType = codegen.fromProperty("type", clubForCreation);

        Assert.assertEquals(clubForCreationType.isReadOnly, false);

        // NOK
        Schema clubForUpdate = openAPI.getComponents().getSchemas().get("clubForUpdate");// clubForUpdate.getProperties().get("type")).getReadOnly() is null
        CodegenProperty clubForUpdateType = codegen.fromProperty("type", clubForUpdate);

        Assert.assertEquals(clubForUpdateType.isReadOnly, true);
    }

Related issues/PRs

Suggest a fix

When a property is a ComposedSchema (espacially an "allOf") it should sync/merge all readOnly / description / ... values...

Start with src/test/resources/3_0/allOf-readonly.yaml and testAllOfReadonly in DefaultCodegenTest. Trace DefaultCodegen.fromProperty for the ComposedSchema using allOf, then verify that the test distinguishes the ordinary property from the readOnly allOf property. Done means the provided assertions pass, including isReadOnly=false for clubForCreation and true for clubForUpdate.