Updated version of Signing changes

[Penguinwizzard]

This is a rebase of johnmcpms/signing on the latest version of microsoft/msix-packaging@master, with a couple of small patches to get it working (and handling pfx passwords). Ideally, with a simpler history and a few of the merge fixes rolled in, this should be an easier change to land than the existing branches where multiple contributors have done repeated merges of master into the topic branch.

[Penguinwizzard]

I've now added some automated tests, albeit fairly simple ones - they pack an MSIX, sign it, and then validate its signature and contents.