[kotlin] [client] Implement inheritance and oneOf support in kotlin-client

Created by: vlsergey

Add example based on oneof_polymorphism_and_inheritance.yaml schema and fix all problems that prevented to correctly implement it:

• Change kotlin-client generation to support three different model types: data class, open class and interface
• Correctly implement parent constructor call (from open / data class to parent open class)
• Add implemented interfaces list (i.e. supports oneOf interfaces)
• Add override flags so kotlin code can be compiled now
• Add inheritedVars collection to CodegenModel thus we can correctly handle case when property specified both in current and parent model

I do believe that it is a non-breaking change for compilable code generation, but it surely will change how non-compilable kotlin code generated (and makes it compilable)

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh

  Commit all changed files. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*. For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master (6.1.0) (minor release - breaking changes with fallbacks), 7.0.x (breaking changes without fallbacks)
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