Java client, adds header

[spacether]

Java client, adds (response) header feature

• header serialization + de-serialization added
• header de-serialization integrated into response de-serialization
• header de-serialization integrated into responses de-serialization
• documentation generated for component headers
• java generator feature documentation updated to show response + header features

Future Work

• response docs list the headers, link to the headers schema docs

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.