Implement a minimal interface for reporting spent ZKAPs to a spending service.

Created by: tomprince

This includes a minimal in-memory implemenation which can be used by tests, and adds the appropriate calls to the new interface to the storage server implemenation.

This is based on #269. I have a work in progress real implementation in #270 which talks to https://whetstone.privatestorage.io/privatestorage/zkap-spending-service/-/merge_requests/4.