add command to list all available runtimes, and show the default

This data can be extracted from the json from a GET call to https://adobeioruntime.net

The default runtime is important for users to minimize cold starts/optimize latency. See https://developer.adobe.com/runtime/docs/guides/using/throughput_tuning/#using-pre-warm-containers-or-optimizing-against-cold-starts

List of runtimes and packages installed by default are here as well: https://developer.adobe.com/runtime/docs/guides/reference/runtimes/