Support for alternative crate registries

[dsteeley]

This crate currently only supports querying crates.io. It would be really useful if it could query alternative crate registries as implemented in rust-rfcs-2141.

It would also need to support authentication as detailed in 3139-cargo-alternative-registry-auth

I'm happy to make this contribution.

[theduke]

Sure.

The client is already using a base_url , so all that's needed is a way to provide a custom base.

I'd appreciate a PR.
(just make sure to update both the sync and the async version)