Emit setup script output from ISetupRunner

[ranweiler]

Summary of the Pull Request

Change the internal ISetupRunner interface to return a serializable representation of the script process output. In a future PR, we will consume this to reflect script failure in a new scheduler state transition.

PR Checklist

• Applies to work item: On task setup script failure, tasks should be marked as "failed" and nodes should be re-imaged #13
• CLA signed. If not, go over here and sign the CLI.
• Tests added/passed
• Requires documentation to be updated
• I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #xxx

Validation Steps Performed

• Tested manually