[Kotlin-Spring] add Kotlin based Spring Cloud OpenFeign generator

[gr4cza]

Add Kotlin based Spring Cloud OpenFeign Client generator. It is based on the already existing JavaSpring spring-cloud library.

e.g. --library spring-cloud via CLI

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.
• In case you are adding a new generator, run the following additional script :

  ./bin/utils/ensure-up-to-date.
  

  Commit all changed files.
• File the PR against the correct branch: master (6.6.0) (minor release - breaking changes with fallbacks), 7.0.x (breaking changes without fallbacks)
• 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 (2017/09) ❤️, @dr4ke616 (2018/08) @karismann (2019/03) @Zomzog (2019/04) @andrewemery (2019/10) @4brunu (2019/11) @yutaka0m (2020/03)

[gr4cza]

@wing328 can you please take a look on this one?

[gr4cza]

I would appreciate if some of you could take a look on it @jimschubert (2017/09) ❤️, @dr4ke616 (2018/08) @karismann (2019/03) @Zomzog (2019/04) @andrewemery (2019/10) @4brunu (2019/11) @yutaka0m (2020/03). I would really love to use this with the next version.

[JanGroot]

Love the addition, I am a little confused though.
I'd expect a client to be part of the main "kotlin" generator, and spring-cloud then as one of the options beside the
jvm-http4, jvm-retrofit etc.

Now it's part of the server code and ALSO generates the client.

@gr4cza