Support alternate registries

[bonsairobo]

For example, shipyard.rs.

It seems like any registry server implementing the git protocol should be supported with a small amount of configuration. But I can't say how difficult this would be to implement or maintain.

[bonsairobo]

Ah I just found this: https://docs.rs/crates-index/latest/crates_index/struct.Index.html#method.from_url

I'll see if I can make this work.