[kotlin][moshi] add polymorphic support

Created by: kvn-stgl

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"

Saddly in the current petstore samples are no discriminator, that's why I didn't add a sample. My plans for the future are to directly change the interface to a sealed class. Any recommendations for the strategy therefore?

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