[REQ] [kotlin-client] Support Enum Fallback Values

Is your feature request related to a problem? Please describe.

When an unknown value of a string pops up to a generated kotlin client, the serialization fails.

A conversation on the general spec is here, but is ongoing.

Various clients have implemented this for reliability sakes. Eg for:

• dart
• java (see the enumUnknownDefaultCase property)
• symphony

I'm requesting that this be supported for the kotlin generator, which has no such config option

There may be a way to use it, however it is not documented and I'm unable to get it to work.

Describe the solution you'd like

Add an option to allow enum parsing similar to the 3 mentioned above not to fail when finding unknown values. See other client implementations & copy your ideal solution.

Describe alternatives you've considered

A modification to the OpenApi Specification is ongoing in consideration, but the preferred approach is still to have specific clients handle the use case.

Additional context

N/A

Start with modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/KotlinClientCodegen.java around line 602 and review docs/generators/kotlin.md for existing options. Compare the referenced Java and Dart implementations and the ongoing OpenAPI Specification discussion. Done means the Kotlin generator exposes a documented option for fallback handling of unknown enum values and the generated client no longer fails on them.