Break out Artifactory promotion into a separate task

[gregturn]

Right now, staging and promotion are done all at the same time for an Artifactory release.

If we could capture all the necessary metadata and leave the user with either A) instructions or B) commands to execute (or some other CI mechanism), it would be nice to better align Artifactory staging with Maven Central staging.

Maven Central staging is very easy to drop the release if something went wrong and simply try again, while Artifactory is arduous to do the same. That's because Artifactory doesn't afford us a single "build" record.