Add support for oneOf with discriminator when using kotlinx.serialization

[diastremskii]

This PR adds support for oneOf with discriminator when using kotlinx.serialization library.

Approach taken does tagged union emulation by adding value classes wrapping the original entities.

The spec included for testing attempts to cover tricky use cases with oneOf:

• oneOf with discriminator itself
• Single entity reused in multiple oneOf’s (sort of multiple inheritance)
• Different discriminator names taken from the same child entity
• Different mapping values when using the same entity
• Children with property that has the same name, but different types (making sure it's not hoisted to parent)

PR checklist

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/cc @karismann @Zomzog @andrewemery @4brunu @yutaka0m @stefankoppier @e5l as technical committee

[wing328]

please add the new folder to the gihtub workflow so that it will be tested moving forward

https://github.com/OpenAPITools/openapi-generator/blob/master/.github/workflows/samples-kotlin-client.yaml

[diastremskii]

Ahh I see, thank you! Updated the workflow!

[diastremskii]

Hmmm not sure how can I rerun the build, seems like it failed for unrelated reasons, given that it was green for previous commit, and I've only updated the yaml

[diastremskii]

Ah I think that's CloudFlare being down: https://www.cloudflarestatus.com/

[wing328]

restarted failed jobs and all passed

[wing328]

thanks for the contribution, which has been merged

[diastremskii]

Thank you so much for a quick review!