Add support to export Thresholds

[xendren]

With K6, using thresholds to indicate potential issues with load is a common practice with the load testing. Looking at the k6 summary in our logs, show whether the thresholds passed or failed. When using the prometheus output, the thresholds are not pushed up into prometheus, or at least we are not able to find them. This seems to be a big piece of testing information that is missing if we choose to output to Prometheus. We did try using the xk6-output-timescaledb output, and it writes the information to a thresholds table that is used in the sample dashboard. We prefer using PromQL, so we would like to be able to see the thresholds in Grafana. I believe we can manually setup thresholds in the dashboard, but that would be a static threshold that can't be defined in our k6 test script or option file. Are we missing something obvious, or are there plans to add thresholds in the future?