[BUG] [cpp-ue4] Missing AnyType declaration for any-type objects

Created by: madrazzl3

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

The UE4 code generator produces invalid code (code that cannot be compiled) for arbitrary-type objects:

#pragma once

#include "BaseModel.h"
#include "AnyType.h"

 ...

AnyType MyValue;

 ...

Neither a header with name AnyType.h is generated nor type AnyType declared anywhere in a project.

openapi-generator version

master - 5.3.0

OpenAPI declaration file content or url

Full scheme

arrayOfSomething:
          type: array
          items: {}
          nullable: true

Generation Details

openapi-generator  generate -i https://gist.githubusercontent.com/madrazzl3/da2502b6816aaa98713cba75e595f373/raw/d53da8c6098167768c3e5b1ad1381467397d6a51/example-scheme.yaml -g cpp-ue4 -o generated-client/ --additional-properties=cppNamespace='api',unrealModuleName='MyAPI',modelNamePrefix=''

Steps to reproduce

1. Generate UE4 client with any of available tools
2. Try to build generated module

Related issues/PRs

Similar problem with other generators:

• #10036 (closed) Kotlin
• #7618 Swift (also #7617 (closed))
• #9969 C
• #7458 C#
• #6332 (closed) Typescript
• #6668 (closed) PhP

Suggest a fix

Add type mapping AnyType -> TSharedPtr<FJsonValue> to keep ability to parse fields later.