[kotlin-spring] interfaceOnly=true flag skips spring-boot plugin repackage goal

[zaenk]

As interfaceOnly=true not generates main class the repackage goal for the artifact fails.

This change applies the same template as used in JavaSpring or in kotlin-spring spring-cloud library.

You can reproduce this using a minimal sample demonstrated in this repository https://github.com/zaenk/kotlin-spring-interface-only

• this repo generate the code using npx @openapitools/openapi-generator-cli generate -i openapi.yaml -g kotlin-spring --additional-properties=interfaceOnly=true command
• you can observe the faulty behaviour running mvn repackge after the code is generated. For reference here are the relevant logs:

------------------------------------------------------------------------
[INFO] Total time:  7.281 s
[INFO] Finished at: 2023-05-12T17:50:06+02:00
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.springframework.boot:spring-boot-maven-plugin:2.6.7:repackage (repackage) on project openapi-spring: Execution repackage of goal org.springframework.boot:spring-boot-maven-plugin:2.6.7:repackage failed: Unable to find main class -> [Help 1]
[ERROR] 
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR] 
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/PluginExecutionException

Why is this important?

1. it will keep parity with the behaviour with generators JavaSpring and kotlin-spring spring-cloud library
2. makes it possible to generate interfaces and models and packaging them in a JAR as a SDK

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: this change not affects the samples, does not introduces new config

  ./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 : ~~ not a new generator

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

  Commit all changed files.
• File the PR against the correct branch: master (6.3.0) (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)

[4brunu]

Looks good to me 👍