CLI tool to facilitate populating ZKAPs

[meejah]

While writing a document discussing how to use some experimental new features in magic-folder, we identified that it would be nice to have a CLI tool to accomplish the steps required to populate the ZKAP database with valid tokens.

For Private Storage specifically, this would do approximately what the GUI does but without needing to launch a Qt + Python GUI and a Web page. This would make sense for humans who don't want to use a GUI or for automated / headless systems like CI machines.

These steps include (but might not be limited to):

• create a voucher
• use Strip to pay for that voucher
• redeem the voucher at Private Storage's payment server
• put the resultant ZKAPs in the database

Mostly this means calling some HTTP APIs on the ZKAPAuthorizer running in a Tahoe-LAFS client on the local machine. The "use Stripe" part might be accomplished via https://stripe.com/docs/stripe-cli/overview at least for "staging" / non-production cases. For production, there might not be a way around the "use a Web browser" part. (These parts require investigation).