Graceful shutdown

Spectroperf should handle interrupt signals and gracefully shutdown. Sometimes mid-run we have the results that we need and it would be good to stop spectroperf while still getting the artefacts from the runs. To do this we should intercept the interrupt signal and maybe enter the ramp down period, or just stop running the workload and output the metrics.json file.

No files or tests are named. Start at the workload run entry point and trace interrupt handling, ramp-down, and metrics.json artifact generation; done means an interrupt stops or ramps down the workload while still producing the required artifacts.

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