AppUpdater: Add ability to include metadata files in diffs

Branched from https://github.com/endlessm/xdelta3-dir-patcher/pull/11 so don't merge until that PR is included.

[endlessm/eos-shell#3985]
@dbnicholson Without a hardcoded location for the file, someone might be tempted to place the file in the "xdelta" dir, causing all kinds of havoc and I'm a fan of convention over configuration. Either way, the repo is public so if someone wants this custom functionality, we aren't stopping them.