Defend against the length-contraction slot_testv_and_readv_and_writev DoS

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

Fix certificate validation errors during environment setup on CI

maybe this nix-env invocation will work

hopefully nix-shell behaves a little more predictably than nix-env wrt NIX_PATH

maybe 2.3.16, a very recent 2.3, will do better.

that version we were on was pretty darn old, for sure

Handle passes using the `Pass` model object in more places

Make generating cross-plaform pins easier.

Convert `Pass`, `UnblindedToken`, `RandomToken` and `Voucher` to use bytes internally.

The later function takes a count of passes to generated, and passes
them to `make_passes`. This also changes `make_passes` to return `Pass`es,
which is what `get_passes` wants to receive.

Record server-side Prometheus metric for spent ZKAPs

also rename and clarify the read_bucket `size` parameter

The test demonstrates this now by depending on part of the successful result
from the underlying storage server object.

It's more specific and less powerful, therefore more expressive and safe.

Co-authored-by: Tom Prince <tom.prince@private.storage>

Make the lease duration config reader respect the lease crawler config

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.