[kotlin][moshi] add polymorphic support with sealed classes

Created by: kvn-stgl

This is almost the same PR as #9159 but it also adds support for sealed classes together with moshi. It will also resolve the issue #8059. I created a separate PR because the changes of my old PR are mostly in the templates (and straight forward). I think this one is a bit more complex, so it make sense to merge the other one first.

Currently it's not possible to use Kotlin and Moshi with the oneOf-Keyword. I tried to implement the PolymorphicJsonAdapterFactory to support this feature. The reason why I set the target branch to master is because the current strategy will fail during runtime (it's not allowed with moshi to serialize a interface directly).

I tested it with the following config:

generatorName: kotlin
outputDir: samples/client/3_0/kotlin-retrofit2-moshi-polymorphic
library: jvm-retrofit2
inputSpec: modules/openapi-generator/src/test/resources/3_0/allOf.yaml
templateDir: modules/openapi-generator/src/main/resources/kotlin-client
additionalProperties:
  serializationLibrary: moshi
  artifactId: kotlin-retrofit2-moshi-polymorphic
  moshiCodeGen: true
  useSealedClass: true

Saddly in the current petstore samples are no discriminator, that's why I didn't add a sample.

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