Allow to pass registry ssh key

[pietroalbini]

Hello, and thanks for the PR!

The code changes look good, but I'm worried about the extensibility of the API, as git2 supports more authentication methods than just in-memory SSH keys. I think the most extensible solution would be to make the AlternativeRegistry struct public, with an API similar to this:

use rustwide::{AlternativeRegistry, Crate};

let mut registry = AlternativeRegistry::new("https://github.com/rust-lang/crates.io-index");
registry.authenticate_with_ssh_key("key");

let krate = Crate::registry(&registry, "name", "version");

[l4l]

Oh, it seems I missed a notification for that thread, sorry for that.

AlternativeRegistry as a pub item makes a perfect sense, although it seems to be a breaking change. Is it fine?

UPD: it seems current changes are breaking anyway due to new Crate::registry arg. So I just proceed with the proposed option.

[pietroalbini]

Sounds good! We're also pre-1.0 on rustwide, so it's fine to make breaking changes, especially to make the API easier to extend in the future!

[jyn514]

@pietroalbini I think this is waiting on your review.

[gregvirgin]

I'm currently working with cloudsmith.io, which will only support git credentials. We're configuring our own image to get around this, but want to see if this method could support adding to .git-credentials on the image as well.

[pietroalbini]

Sorry for the late merge, will release this soon!

[pietroalbini]

Released rustwide 0.15.0 with this change.