Fold groovy-sandbox into script-security-plugin

[gbhat618]

This PR merges the https://github.com/jenkinsci/groovy-sandbox into script-security-plugin (here) as this is the single consumer(*) and the sole way of shipping any changes in groovy-sandbox. Keeping the groovy-sandbox added friction in delivering bug fixes. Especially security fixes needed very careful attention to stage the groovy-sandbox release, and then consume it in script-security-plugin and stage script-security-plugin release.

This PR folds the groovy-sandbox library into a folder here with the same name groovy-sandbox/. The existing script-security-plugin source code is moved to plugin/

(*) Though there is workflow-cps/lib/ which is one other (optional) user for groovy-sandbox this change still publishes groovy-sandbox so it should be no changes to consumer.
https://github.com/jenkinsci/workflow-cps-plugin/blob/30c8c00684a37764a1083dd40b496ccdccc90dd7/lib/pom.xml#L58-L63

Procedure

The following 3 commits are the important, below are details of how they are produced.

• commit1 182212e : moved the script-security-plugin source code into plugin/ folder, introduced the parent pom.xml and adjusted the relationship between plugin/pom.xml. mvn clean package works.

   mkdir plugin
   git mv src       plugin/src
   git mv pom.xml   plugin/pom.xml
   git mv README.md plugin/README.md
  
   # adjust the pom.xml
  
   git add .
   git commit -m "Restructure into a multi-module project"
  

• commit2 8d0f97c : prepares by coping the groovy-sandbox repo history into groovy-sandbox/ doesn't fix the module yet.

  git remote add groovy-sandbox-upstream https://github.com/jenkinsci/groovy-sandbox.git
  git fetch groovy-sandbox-upstream
  git merge -s ours --no-commit --allow-unrelated-histories groovy-sandbox-upstream/master
  git read-tree --prefix=groovy-sandbox/ -u groovy-sandbox-upstream/master
  git commit -m "Merge groovy-sandbox history into groovy-sandbox/ subdirectory"
  

• commit3 1f81ac0 : removes unneded files from groovy-sandbox/ and adjusts all 3 pom.xml to make it locally built, and consumed.

  git rm -r groovy-sandbox/.mvn
  git rm -r groovy-sandbox/.github
  git rm groovy-sandbox/Jenkinsfile
  git rm groovy-sandbox/.gitignore
  
  # fix all 3 pom
  
  git add .
  git commit -m "Integrate groovy-sandbox as a module of script-security-plugin"
  

Checklist

• Verify this PR is actually preserving history and commits correctly
• jenkins-infra/repository-permissions-updater PR
• jenkinsci/groovy-sandbox remove source code and update README
• Verify PCT test run passes
• Push old groovy-sandbox version tags into this repo
• Archive jenkinsci/groovy-sandbox on GitHub

Testing done

mvn clean verify passes, all tests of both groovy-sandbox and plugin are executed.

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed