Java, adds enums and uses them in Schema validate methods

[spacether]

Java, adds enums and uses them in Schema validate methods
Updates

• Schema.getSchemas refactored into generator method
• Java schema docs improved, only list array/map builder and output classes in summary if those exist in getschemas. Also conditionally lists enums of they exist
• Java, removes unused imports from schemas

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.