[JENKINS-73421] Expose getExternalizableId() in RunWrapper API

[stuartrowe]

Implementation for https://issues.jenkins.io/browse/JENKINS-73421.

Expose getExteralizableId() in RunWrapper API. Returns the value of the existing externalizableId member which is assigned the externalizableId of the underlying build object when the RunWrapper is constructed.

Testing done

Added new unit test in RunWrapperTest.

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 demonstrates feature works or fixes the issue

[stuartrowe]

@jglick are you able to review this please?

[r00t-]

great, thanks!
(this resulted from my comment at jenkinsci/pipeline-build-step-plugin#106 (comment) )