Java client, adds RequestBody

[spacether]

Java client, adds RequestBody

• request body supports application/json and text/plain content type serialization
• request bodies use sealed classes because there can be multiple pairs of contentType, validatedSchemaPayload that are accepted
• Sealed validated payload output classes added to schemas to describe the validatedSchemaPayload that request bodies need
• tests added of json and plain text request body serialization

Future Work

• add bytes and file handling
• add "application/x-www-form-urlencoded"
• add multipart/form

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.
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  For Windows users, please run the script in Git BASH.