Install Nightly SDK Builds with DNUP

[nagilson]

The prototype of DNUP can install sdks or runtimes based on the release manifest. But what about the nightly builds that are often used internally? Those will have signatures as well that we need to verify, but that can be done in the download content step, as the archive signatures should be relatively similar.

We need to write out the UX for installing a nightly. How does it get tracked in a manifest, and how do users install it?
Is it as such?
dnup install _nightly_url
Or can I just provide the nightly version?
dnup install nightly_version

I assume we don't update specific nightlies. Please work with our PM before implementation as we have not agreed on the complete solution.