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

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

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[rhys-rant]

Is there any scope for progressing with these changes? We're hitting up against a problem with generating an API client from a spec that uses a discriminator and oneOf for representing two discrete cases, but the code we're being generated merges the properties instead.
Seems like exactly what sealed classes would solve, especially given how the Swift generator generates an enum with associated cases for each type in the oneOf.
It'd be a massive help if we could have access in any way to generating sealed classes instead of the generator merging types!

[rhys-rant]

Is there any chance of getting this functionality merged?

[wing328]

@rhys-rant do you mind filing a new PR based on this one with resolved merge conflicts?

[mattiasback-ps]

I also would like to see this merged asap.