add uuid package attribute

There's been a few times now where my studio would benefit from a released package being assigned a unique ID. Most recently, I need this in order to then identify a transfer job which syncs the package to our studio locations. You could generate your own ID from the package's attributes of course, but having a native ID already assigned solves a few interrelated issues.

I'll probably call this `release_id` to reflect the fact this ID represents the particular release of that particular package. So for eg, foo-1.0.0 being released to /pkgs/internal repo at time X would get one ID; at time Y would get another (perhaps the same pkg version had to be forcefully rereleased); and a release to a different repo would get yet another ID.

The issue names no files, tests, or entry points. Start by locating where Rez assigns and persists metadata for released packages, then clarify whether release_id must vary by repository and by forced rerelease. Done means each package release has a stable unique ID with the distinctions described in the issue.

Written by the indexing model from the issue text.