framework/output: add accessors for metrics and artifacts
framework/output: add accessors for metrics and artifacts

- Add get_metric methods to Result and Output
- Add metrics and artifacts properties to Output

ARM-software/workload-automation,lisatn/workload-automation,ARM-software/workload-automation,setrofim/workload-automation,ARM-software/workload-automation,ARM-software/workload-automation,lisatn/workload-automation,setrofim/workload-automation,setrofim/workload-automation,lisatn/workload-automation,lisatn/workload-automation,setrofim/workload-automation