Add Mergebot features

[mig42]

This PR brings the functionality currently located in the Mergebot plugin.

Installing the plugin will enable DevOps workflows from Plastic SCM to be triggered in Jenkins. Mergebots trigger builds, setting the spec to build in a specific parameter. The Mergebot SCM picks that up and uses it to checkout the sources.

The benefit of this merge is keeping a single codebase for both SCM plugins, and unlock the latest features in the Mergebot plugin: cm as auto-installing tool, setting per-project credentials, cleanup modes.

This PR is still a draft. Pending to do:

• Manual testing
• Update documentation

Testing done

There are a few unit tests added, and the functionality was verified via manual testing.

Tests include:

• Verify that the Plastic SCM VCS plugin is still working as before
  • Freestyle projects
  • Pipeline projects with hard-coded script in project config
  • Pipeline projects from SCM
• Verify that the new Mergebot VCS plugin works as expected
  • Freestyle projects
  • Pipeline projects with hard-coded script in project config
  • Pipeline projects from SCM
• Verify that installing the plugin in an existing environment doesn't break the configuration

Submitter checklist

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