[Java] Feature/wrapped types and default values

Created by: cachescrubber

POC - generic wrapper data type

Enhance CodegenProperty to support Wrapper Types

    public String wrapperType;
    public String wrapperFunc;
    public boolean isWrapped;

Wrapper Types are for Example java.util.Optional or org.openapitools.jackson.nullable.JsonNullable.

By adding generic support for such types we avoid clumpsy and difficult to read templates - Mustache templates are advertised as Logic-less templates.. for a reason ...

PR checklist

• [x ] Read the contribution guidelines.
• [ x] 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.
• [x ] 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.
• [ x] File the PR against the correct branch: master (5.3.0), 6.0.x
• [ x] If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.