zcash_client_backend: Add Arti-based Tor client for fetching USD/ZEC exchange rates

[str4d]

We want wallets to be able to fetch the latest USD/ZEC exchange rate without revealing their IP address to the data sources.

In testing, it's been possible to do this using Arti, which is now usable as a client library. We should add this to zcash_client_backend behind a feature flag, so it can be shared across any wallets that use the backend.