normalizer REPLACE_ONE_OF_BY_DISCRIMINATOR_MAPPING

[jpfinne]

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.

---

Summary by cubic

Adds a new OpenAPI normalizer option to replace oneOf with discriminator mappings when a discriminator is present. This makes polymorphism clearer and improves generated models for Java and Spring.

• New Features
  • New normalizer flag REPLACE_ONE_OF_BY_DISCRIMINATOR_MAPPING=true: for schemas with a discriminator, move oneOf $refs into discriminator.mapping (if missing) and remove oneOf.
  • Docs updated with a CLI example showing how to enable the rule for spring.
  • Tests added for Java client and Spring codegen using GeoJSON samples to verify base type uses JsonSubTypes and child classes extend the base.

^{Written for commit bf1dc5f. Summary will update on new commits.}