[Kotlin] [Feature] Add config to have internal visibility modifier added to all generated files

[wittlock]

Description

Kotlin has a nice feature with it's internal visibility keyword to make classes only visible within the current module of the application. I quite like using it for generated code, which can be plentiful, to prevent it from bleeding into the rest of the project, keeping autocomplete less cluttered.
Adding internal to generated Kotlin classes does not break java interoperability but they do get slightly mangled names to make them more clearly unintended for use, see documentation.

openapi-generator version

5.4.0

Suggest a fix/enhancement

I'm thinking this would be a fairly simple and small thing to add as a first contribution of my own to this project. So I'm planning on offering up the addition myself but going by the contributing guidelines I'm writing an issue about it first to see if there's feedback or objections to be had first.
Config option name suggestion would be simply internal or internalVisibility and default to false.

[mustafaozhan]

Looking forward to see this as well!

[soniaswer]

Hi there, I was wondering if it has been resolved yet or not? I have the same requirement, where I am supposed to add internal keyword to classes, and if possible to classes only in certain directory. So, was wondering if it was ever fixed. In case yes, what's the config param?

[arcadii-meister]

Apparently, this feature has been available for some time, thanks to this PR. It does exactly what you are proposing by setting the internal visibility modifier to Kotlin classes:

openApiGenerate {
    generatorName = "kotlin"
    ...
    additionalProperties = mapOf("nonPublicApi" to "true")
}

[wittlock]

Yeah, that looks like exactly what I was looking for. Closing this issue as it's now redundant.