[kotlin-client] add ignoreUnknownKeys for kotlinx serialization

Created by: kassim

fixes https://github.com/OpenAPITools/openapi-generator/issues/13013

initialises the Json instance for kotlinx serialization with ignoreUnknownKeys = true so that deserialisation doesn't fail when new keys are added to the API

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 (6.1.0) (minor release - breaking changes with fallbacks), 7.0.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.

@jimschubert (2017/09) ️, @dr4ke616 (2018/08) @karismann (2019/03) @Zomzog (2019/04) @andrewemery (2019/10) @4brunu (2019/11) @yutaka0m (2020/03)

docs/generators/kotlin.md

@@ -36,7 +36,7 @@ These options may be applied as additional-properties (cli) or configOptions (pl
 |parcelizeModels|toggle "@Parcelize" for generated models| |null|
 |requestDateConverter|JVM-Option. Defines in how to handle date-time objects that are used for a request (as query or parameter)|<dl><dt>**toJson**</dt><dd>[DEFAULT] Date formatter option using a json converter.</dd><dt>**toString**</dt><dd>Use the 'toString'-method of the date-time object to retrieve the related string representation.</dd></dl>|toJson|
 |serializableModel|boolean - toggle &quot;implements Serializable&quot; for generated models| |null|
-|serializationLibrary|What serialization library to use: 'moshi' (default), or 'gson' or 'jackson'| |moshi|
+|serializationLibrary|What serialization library to use: 'moshi' (default), or 'gson' or 'jackson' or 'kotlinx_serialization'| |moshi|
 |sortModelPropertiesByRequiredFlag|Sort model properties to place required parameters before optional parameters.| |null|
 |sortParamsByRequiredFlag|Sort method arguments to place required parameters before optional parameters.| |null|
 |sourceFolder|source folder for generated code| |src/main/kotlin|

modules/openapi-generator/src/main/resources/kotlin-client/jvm-common/infrastructure/Serializer.kt.mustache

@@ -115,6 +115,6 @@ import java.util.concurrent.atomic.AtomicLong
     }
 
     @JvmStatic
-    val jvmJson: Json by lazy { Json { serializersModule = kotlinSerializationAdapters } }
+    val jvmJson: Json by lazy { Json { serializersModule = kotlinSerializationAdapters; ignoreUnknownKeys = true } }
 {{/kotlinx_serialization}}
 }

samples/client/petstore/kotlin-retrofit2-kotlinx_serialization/src/main/kotlin/org/openapitools/client/infrastructure/Serializer.kt

@@ -32,5 +32,5 @@ object Serializer {
     }
 
     @JvmStatic
-    val jvmJson: Json by lazy { Json { serializersModule = kotlinSerializationAdapters } }
+    val jvmJson: Json by lazy { Json { serializersModule = kotlinSerializationAdapters; ignoreUnknownKeys = true } }
 }