Added support for beforeAgent property in when statements

[MKITConsulting]

This pull request addresses the missing support for beforeAgent inside when statements, see also issue #227.

Testing done

I've added new test cases and a Jenkinsfile with a nested beforeAgent property inside a when statement to cover the use case that without this PR would lead to the error message mentioned in #227.

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
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• 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

[nre-ableton]

Thanks!