[Kotlin-Spring] Support multiline descriptions

Created by: korthout

This adds support for multiline descriptions for operations in the Kotlin-Spring generator for both regular API generation (i.e. Controller) and interface-only API generation.

Multiline descriptions allow us to use rich text representations, e.g. with Markdown. Note that Markdown-formatted descriptions are rendered nicely in Swagger-UI. I imagine that most openapi consumers will be able (or want to) support Markdown (at some point).

The solution for Kotlin-Spring is rather simple, using Raw Strings to contain the unescapedNotes.

I've added a test case for both the regular API and the interface-only API. The Samples were left unchanged updated after running the steps below.

This pull request resolves this issue in Kotlin-Spring only:

• #4111

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 (6.3.0) (minor release - breaking changes with fallbacks), 7.0.x (breaking changes without fallbacks)
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