[java-micronaut] Generate visitor for subtypes with a discriminator

Created by: auke-

When types which extend a common type and are distinguished based on a discriminator are consumed they are often cast to their specific Java type which results in error prone boilerplate code.

By generating a visitor for those kind of types the various subtypes can be consumed in a type safe manner.

This change is based on the same implementation I made for the Kokuwa.io version of the micronaut openapi plugin.

Note that there is also a pull request for the typescript-angular implementation.

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.
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

@andriy-dmytruk