Java: adds sealed boxed classes for validated Schema payloads

[spacether]

Java:

• Schemas: adds sealed validated payload boxed class for each schema, method validateAndBox creates instances of it
  • these classes are boxed because the data property always holds the validated payload
• those sealed classes will be used as inputs for request bodies, this prevents duplicating the validate interfaces in the request body classes. If this weren't done, then 6 or more validate interfaces would need to be generated in the requestbody for the validate methods. 6 or more comes from 6 json schema types and the more includes enum types.
• adds templates that will be used in future request body work
• schema docs updated to list the sealed class, implementing classes, and the new validateAndBox methods

Future Work

• generate RequestBody classes and use the sealed schema classes in them

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:

  mvn 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/python*.
  For Windows users, please run the script in Git BASH.