[Kotlin][Multiplatform] Support generating gradle module as child module

Created by: tkirshboim

PR Description

The Problem

It is currently not possible to use the Kotlin Multiplatform generator template to generate a Gradle child module. This is a problem for projects that wish to do that.

I created an example project that demonstrates the issue as well as how this change resolves the problem: https://github.com/Airthings/openapi-multiplatform-multi-module

The demo project uses the existing Kotlin Multiplatform Petstore API sample. If you check out the project and try to build it you get the following error:

$ ./gradlew

> Configure project :shared
[ omitted ]

FAILURE: Build failed with an exception.

* Where:
Build file './openapi-multiplatform-multi-module/openapi-generator/samples/client/petstore/kotlin-multiplatform/build.gradle.kts' line: 3

* What went wrong:
Error resolving plugin [id: 'org.jetbrains.kotlin.multiplatform', version: '1.5.31']
> Plugin request for plugin already on the classpath must not include a version

[ omitted ]

BUILD FAILED in 2s

The Solution

Allow omitting the Gradle plugin version declaration by using a new Kotlin generator "additional property" named omitGradlePluginVersions.

To validate that this resolves the issue, you can regenerate the OpenAPI generated KMM code by using the new property from the root folder of the demo project:

(cd openapi-generator && ./bin/generate-samples.sh bin/configs/kotlin-multiplatform.yaml -- --additional-properties=omitGradlePluginVersions=true)

Note

There are no changes to the generated samples because the default value for the new property omitGradlePluginVersions is false. When that's the case, the existing samples are not expected to change.

CCs

Kotlin technical committee: @jimschubert @dr4ke616 @karismann @Zomzog @andrewemery @4brunu @yutaka0m

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 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh

  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*. For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master (5.3.0), 6.0.x
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.