End-to-end with ristretto

Created by: exarkun

This adds a Nix-based system test that creates an issuer and tahoe-lafs client, introducer, and storage nodes. It has the tahoe-lafs client node connect to the tahoe-lafs storage node via the tahoe-lafs introducer and then redeem a voucher with the issuer for ZKAPs. Finally, it uses the ZKAPs to authorize a write to the storage server and then verifies it can read the data back.