Add per-platform download URLs that will always redirect to "latest"

[mike-myers-tob]

Although version 5.0.1 is "current" on the downloads page, the URL https://pkg.osquery.io/windows/osquery.msi will retrieve version 4.9.

Users would like to be able to automatically grab the latest release.

Alternatively if we decide to not encourage or support automatic retrieval of "current" osquery installers, we ought to remove unversioned filename URLs being served at all (i.e., osquery.msi as opposed to osquery-5.0.1.msi).

[directionless]

This isn't a website thing -- it's a packaging thing. I'd have opened this in the main repo. We should probably disable issues here