Custom error messages for size, length, min & max validations in Spring & Spring-Kotlin

[svenallers]

Resolves: #6375

This PR adds the ability to customize error messages on Spring bean validations for size, length min and max for Java and Kotlin. The approach I chose is inspired by the current Micronaut approach (https://micronaut-projects.github.io/micronaut-openapi/latest/guide/).

Apart from that I also added assertion classes that parse the Kotlin code to better test my changes. Those where mainly inspired by the assertion classes currently used for the Java tests.

Added vendor extensions

Extension name	Description
x-size-message	Customizes the invalidation error message for the size or length of a variable
x-minimum-message	Customizes the invalidation error message for the minimum value of a variable
x-maximum-message	Customizes the invalidation error message for the maximum value of a variable

Targeted Generators

• Java Spring: @cachescrubber @welshm @MelleD @atextor @manedev79 @javisst @borsch @banlevente @Zomzog @martin-mfg
• Kotlin Spring: @karismann @Zomzog @andrewemery @4brunu @yutaka0m @stefankoppier @e5l

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 || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in WSL)
  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*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• File the PR against the correct branch: master (upcoming 7.x.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
• If your PR solves a reported issue, reference it using GitHub's linking syntax (e.g., having "fixes #123" present in the PR description)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.