Java generator: adds gradle build tool support

[spacether]

• Adds buildTool cli option for java with the options: gradle/maven (default maven)
  • buildTool is an additionalProperty passed in to the generate command
• Gradle wrapper files are gitignored
• Some small java fixes to satisfy the Java checker checks
• Java readmes updated to show grade/maven instructions
• Java petstore sample switched to using gradle

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