Perform dry-run/storage estimation

I’d really like kopia to be able to perform a dry-run, where it will only run through the data to be snapshotted and calculate the amount of space needed for backup. This would probaly mean, that kopia will have to save the hash values on some external storage, but I wouldn’t care. People who will have storage in the 10ths of TBs to backup will surely be having some SSD space available to spare.

In my example, I am running a Samba server with approx. 400 shares on it - each of them being on a distinct ZFS. The amount of data stored is currently shy of 100 TB and I’d really like to know beforehand, how much savings kopia will be able to squeeze from the data.