[kotlin-client][kotlin-server] feature: allow creation of kotlin data classes that implement java.io.Serializable

Created by: janweinschenker

…rializable, by adding the config option serializableModel=(true|false)

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
• Filed the PR against the correct branch: master, 4.1.x, 5.0.x. Default: master.
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Description of the PR

I added a new configOption "serializableModel=(true|false)" to org.openapitools.codegen.languages.AbstractKotlinCodegen and to the files

• kotlin-server/data_class.mustache
• kotlin-client/data_class.mustache

in order to allow to generate kotlin data classes that implement the interface "java.io.Serializable". This will come in handy if data classes are generated with the intention to store data in databases or in distributed caches (i.e. hazelcast)