#21746 [kotlin-client] Support for integer enums with kotlinx with custom serializer

Following PR https://github.com/OpenAPITools/openapi-generator/pull/21248
Following this issue, when using integer enums with the kotlin-client generator, the generated enum will serialize and deserialize strings, not integers.

How to test

1. Create a test.yml file with the following content:

openapi: 3.0.3
info:
  title: Demo
  version: v1

components:
  schemas:
    Code:
      type: integer
      enum:
        - 0
        - 1
    StringCode:
      type: string
      enum:
        - hello
        - world

paths:
  /do:
    get:
      summary: Do something
      responses:
        200:
          description: Successful response
          content:
            application/json:
              schema:
                type: object
                required:
                  - code
                properties:
                  code:
                    $ref: '#/components/schemas/Code'

1. Create a config.json file to generate only relevant content with the following:

{
  "modelPackage": "com.test",
  "generatorName": "kotlin",
  "enumPropertyNaming": "UPPERCASE",
  "additionalProperties": {
    "serializableModel": true,
    "omitGradleWrapper": true,
    "omitGradlePluginVersions": true
  },
  "globalProperties": {
    "apis": false,
    "models": "",
    "apiDocs": false,
    "modelDocs": false,
    "apiTests": false,
    "modelTests": false
  }
}

1. Build openapi on the head branch of this PR: ./mvnw clean install -DskipTests -Dmaven.javadoc.skip=true
2. Run the builded version against the files you've previously created: java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar generate -i test.yml -g kotlin -o out/ -c config.json
3. See the generated code handling string enums (StringCode.kt) and integer enums (Code.kt)

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
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  ./mvnw clean package || exit
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  (For Windows users, please run the script in WSL)
  Commit all changed files.
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