[BUG][Elm] specifying x-enum-varnames causes malformed output

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

Context: Server sends & expects int values for enum types, we want to use enum names in the Elm code.

Adding x-enum-varnames seems to be the recommended approach, but that leads to malformed Elm code being output.

E.g., in src/Api/Data.elm:

Expected:

intFromYesNoNa : YesNoNa -> Int
intFromYesNoNa model =
    case model of
        YesNoNaUnknown ->
            0

        YesNoNaYes ->
            1

        YesNoNaNo ->
            2

        YesNoNaNa ->
            999

Actual:

intFromYesNoNa : YesNoNa -> Int
intFromYesNoNa model =
    case model of
        YesNoNaUnknown ->
            Unknown

        YesNoNaYes ->
            Yes

        YesNoNaNo ->
            No

        YesNoNaNa ->
            Na

openapi-generator version

5.0.0-SNAPSHOT, from nixpkgs (commit f75c8753d80959f6bf63237748e799849c74179f).
Doesn't seem like a regression.

OpenAPI declaration file content or url

{
  "openapi": "3.0.1",
  "info": {"title": "Hippo API", "version": "v1"},
  "paths": {"/foo": {"get": {"tags": [], "summary": "x", "responses": {"200": {"description": "Success", "content": {"application/json": {"schema": {"$ref": "#/components/schemas/YesNoNa"}}}}}}}},
  "components": {
    "schemas": {
      "YesNoNa": {
        "enum": [0, 1, 2, 999],
        "x-enum-varnames": ["Unknown", "Yes", "No", "Na"],
        "x-enum-descriptions": ["Unknown", "Yes", "No", "N/A"],
        "type": "integer"
      }
    }
  }
}

Generation Details

$ openapi-generator-cli generate -i ./enumnames.json -g elm

Steps to reproduce

1. Generate as above
2. elm make src/Api/Data.elm

---

or, using the latest master:

$ docker pull openapitools/openapi-generator-cli:latest
$ docker run -v /home/rkb/projects/namedenums/:/host -it openapitools/openapi-generator-cli:latest generate -i /host/enumnames.json -g elm -o /host/dckr
$ elm make dckr/src/Api/Data.elm

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/issues/5794

Suggest a fix

Looks like #917 added enum names support to the Java generator, with an updateEnumVarsWithExtensions() method

Start with the enumnames.json example and generate the Elm client using the documented openapi-generator-cli command. Inspect the generated src/Api/Data.elm and run elm make to reproduce the malformed enum conversion. Done means the generated function maps each enum constructor to its integer value and the Elm file compiles.