Add another way to force a file to be marked as uncached in kopia

[ashmrtn]

Mod time alone is not sufficient to allow Corso to get new data into kopia, especially if we want to start picking up data we weren't previously backing up. This ties into how kopia decides a StreamingFile is cached. Adding the ability to control one or more of these fields besides ModTime will give us the ability to force data updates on a per-item basis as needed

Another thing that would be good to have is to not have to read the previous version of an item to determine if we should send the updated version of the item to kopia. That will reduce the number of reads kopia does during a backup and keep the backup logic simple

[ashmrtn]

if we don't want to add another field to the file entry struct that kopia currently uses, hijacking the Owners field for StreamingFile is probably our best bet. It would give us access to two uint32s and is already used in the comparison for cached files