[Kotlin] Use mutable container types when 'modelMutable' is enabled

Created by: rm3l

This should close #11088 (closed).

Basically, when the Kotlin generators are run with modelMutable set to true, the changes here allow to generate read-write Mutable Kotlin container types (like MutableList or MutableMap) for OpenAPI arrays and dictionaries, instead of read-only types (like List or Map). This way, it makes it possible to mutate such containers right away, without performing potentially expensive copies.

See #11088 (closed) for more context and details.

Maybe I missed something, but I noticed that unlike the other Kotlin generators, the kotlin-client one uses the raw templates rather than the Java source files to determine the different data types. Please do let me know if things should be done differently.

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 (5.3.0), 6.0.x
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

@jimschubert @dr4ke616 @karismann @Zomzog @andrewemery @4brunu @yutaka0m Could anyone of you please take a look at this ? Thanks.