Runtime download of necessary resources from trusted registries

Summary of the new feature / enhancement

Leveraging https://github.com/PowerShell/DSC/issues/92 but need similar solution for PSGallery via PSResourceGet DSCv3 resource

A common feedback we've heard from partners and customers is they don't want to package up or install/update resources before deploying a configuration (particularly in a fan-out type scenario). Instead, we would have a way to register registries and at runtime use that to find the appropriate resources, download them (don't need to actually install, although there may be a benefit to cache in case of runtime issues), and use.

Proposed technical implementation details (optional)

DSC engine currently discovers all the requested resources in the config to make sure they are all available before attempting any deployment of the configuration. In this case, the discovery would be integrated into the dsc resource list code path to use registries and then the code that invokes the resource would need to potentially download and then use. For PowerShell based resources, it may make sense to have an extension whereby PSResourceGet can participate.

Start by reading the dsc resource list discovery path and the resource invocation path described in the issue. Trace how registered trusted registries and PSResourceGet participation could support runtime discovery, download, and use; done means resources can be obtained and used during deployment without prior packaging or installation.