Support for enum variable incase of reusable enum class

Description

When an enum is defined as schema it generates new model which can be reused. But when the variable of this class is defined at other places they are treated as nonprimitivetype and isEnum is false hence they can't be handled as enum variables.

openapi-generator version

openapi-generator v3.3.3

OpenAPI declaration file content or url

test yaml spec:
https://hastebin.com/ibororudof.bash

Command line used for generation

java -DskipFormModel=true -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar generate
-i modules/openapi-generator/src/test/resources/3_0/pet_oas3.yaml
-l c -o out/oas3debug

Steps to reproduce

Generate client for C/C++/Python/Java has same result on each generator.

Related issues/PRs

Suggest a fix/enhancement

I tried playing around a bit, I was able to get around it but not sure if it was the correct way.
In DefaultCodegen.java, I added extra variable allDefinitions to addVars function. In main addVars function I check if $ref and if present then if getEnum is not null then i set isEnum to true.

Start with the linked test YAML and the generation command, then inspect DefaultCodegen.java, especially addVars and its handling of $ref values. Reproduce the output for C, C++, Python, or Java and trace why reusable enum references retain isEnum=false. Done means referenced enum variables are identified consistently across the affected generators and covered by an appropriate regression test.