[kotlin-client] Add support for integer enums in serialization step

[CHervaudBetclic]

Following this issue, when using integer enums with the kotlin-client generator, the generated enum will look like this:

@Serializable
enum class SomeCode(val value: kotlin.Int) {
    @SerialName(value = "0")
    _0(0)
}

You can see the values of the enum members are not integers, they are strings (value = "0").
Using integer enums, it's should look like this:

@Serializable
enum class SomeCode(val value: kotlin.Int) {
    @SerialName(value = 0)
    _0(0),
}

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'

2. 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
  }
}

3. Build openapi on the head branch of this PR: ./mvnw clean install -DskipTests -Dmaven.javadoc.skip=true
4. 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
5. 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.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in WSL)
  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• File the PR against the correct branch: master (upcoming 7.x.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

Hello all @dr4ke616 @karismann @Zomzog @andrewemery @4brunu @yutaka0m @stefankoppier @e5l, could you take a look please? 😁

[4brunu]

@CHervaudBetclic did you tested this locally?

[CHervaudBetclic]

@4brunu Hello
Yes, following the How to test section of the PR description

[4brunu]

Thanks, looks good to me 👍

[CHervaudBetclic]

@4brunu Do you have any idea about the next release date?
I don't know if you have planned releases are if they are just done on the fly

[4brunu]

You can find the next planned release date on the readme.
https://github.com/OpenAPITools/openapi-generator?tab=readme-ov-file#11---compatibility

[Him188]

This change is incorrect and breaks compilation.

SerialName only takes String arguments:

public annotation class SerialName(val value: String)

Passing an integer literal is a compilation error.

[hrach]

@wing328 @4brunu please, revert this; it was not tested as it doesn't compile at all.

[4brunu]

This PR was reverts #21491