Java, improves map output type

• input types used in SomeSchema.validate
• input types used in SomeSchemaMap.of
• output types used in SomeSchema.getMapOutputInstance
• adds accessor methods for validly named properties + required properties
• adds accessor method getAdditionalProperty

Notes:
If an optional key/additional property key is not present an exception is thrown

• this was done because null is not an allowed value for an unset key. Null may be set as a value in a required or optional property, it does not mean unset in this context

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.