[JENKINS-28924] Support for non-java projects

[Vlatombe]

https://issues.jenkins-ci.org/browse/JENKINS-28924

This picks up #15 where it was left, making it extensible as well as refactoring the existing java support to estimate tests from files, leaving room for future generic or specific implementations to support other languages.

This introduces a new extension point, TestMode, allowing to specify test entities (classes or cases) to parallelize on. It provides as well the ability to return an estimate obtained from the workspace, if no historical result was provided.

• Remove estimateTestsFromFile, this is now covered automatically by the chosen TestModeimplementation, if it supports it.

Various demos updates:

• Multi-stage build to obtain maven, should ease further updates
• Removal of central mirror from the settings.xml as the Jenkins infrastructure no longer mirrors it.
• Put Jenkins in debug mode automatically on startup (port 5005)
• A python sample project. First build runs serially, further builds do parallelization as expected.

Testing done

Ran demo project and verified that both the java and python projects are behaving as expected.

### 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
- [ ] Link to relevant pull requests, esp. upstream and downstream changes
- [ ] Ensure you have provided tests - that demonstrates feature works or fixes the issue

[jglick]

the position of the help handle

True. Was somewhat better in https://github.com/jenkinsci/jenkins/pull/5417 but seems to have wandered since then after one or another core layout refactoring. Affects fields not touched by this PR.