[KotlinSpring] Generated model is invalid if 'inheritance' is used

Created by: mikezx6r

Description

The Kotlin Spring generator currently generates inheritance of data classes if a schema/type uses allOf and $ref. Kotlin data classes do not support inheritance, so this is invalid code.

The generator will need to generate properties from the main object, as well as the sub-object directly.

openapi-generator version

3.2.3, 3.3.0 and 3.3.1-SNAPSHOT

OpenAPI declaration file content or url

https://gitlab.com/mikezx6r/openapi-test/blob/master/src/main/resources/api.yml

Command line used for generation

Using Gradle. kotlin-spring language. Refer to https://gitlab.com/mikezx6r/openapi-test/blob/master/build.gradle for specifics.

Steps to reproduce

./gradlew oAGenerate and view the models generated into the src/main/generated-kotlin folder

Related issues/PRs

Suggest a fix/enhancement

The data class needs to contain all the properties from itself as well as from the 'child' class directly.