* kotlinx serialization fixes
- added new config with kotlinx, discriminator (/w custom name) and
kotlinx_serialization
- remove discriminator properties from the generator in both base and
derived classes
- set discriminatorValue in additionalProperties of derived classes
- add JsonClassDiscriminator the derived classes in the template
- set SerialName to discriminatorValue in the template
- change base classes to sealed class instead of interface
- make variables in base classes abstract
* Generated kotlin-allOff-discriminator-kotlinx-serialization sample
* Added test for kotlinx_serialization with discriminator
* renamed yaml
* Added new sample to github workflow
* Added comments to KotlinClientCodegen::postProcessAllModels
