[kotlin][client] Add inheritance to Kotlin Client templates

[mtraynham]

Adds inheritance to the Kotlin Client codegen templates using interfaces as the parent class.

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• File the PR against the correct branch: master, 4.3.x, 5.0.x. Default: master.
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

@jimschubert (2017/09) ❤️, @dr4ke616 (2018/08) @karismann (2019/03) @Zomzog (2019/04) @andrewemery (2019/10)

[jimschubert]

@mtraynham thanks for keeping updated with master. That's much appreciated. My only concern about the newly added code is the empty newlines it introduces for noop conditions.

I wonder if we can target 4.3.x with this new code? I have concerns that the introduction of the new template files may cause issue, and just to be safe we could release in the minor version which is "breaking change with fallback". The fallback being users could use 4.2.x templates if they have issues.

Any concern with retargeting? As an alternative we could get the vendor extension into master and the template changes against the 4.3.x branch.

[mtraynham]

Thanks @jimschubert for taking a look! I'm not against flattening the new lines, it just makes the template kinda of unreadable. I'll rebase against the 4.3.x branch as well.

[mtraynham]

@jimschubert , I corrected the issues you mentioned and rebased my commits on to OpenAPITools/4.3.x.

Looks like the checks are failing because it's attempting to merge the feature branch into master and getting some merge failures. I can rebase again when those are corrected if need be.

[jimschubert]

Verified inheritance after testing a merge of master. Looks good!