Remote Custom Toolchains

Describe the problem you are trying to solve
It would be nice to be able to specify custom toolchains at a remote location, so that you can very easily download and install custom toolchain using Rustup. So that other distributors can just provide URLs to their distribution, and not require users to manually download and link the toolchain themselves. Of course this remote location would have to follow the same conventions as Rust's CI, but that's not really an issue since most users of this feature would distributing using x.py anyway, just to their servers.

Describe the solution you'd like

rustup toolchain install --custom custom-channel https://custom.example.org

Unresolved Questions

• link is also another command for creating a custom toolchain but is filesystem only right now. Should the link command be used instead of install?
• How would updating a custom channel work? (I think having to uninstall and install manually would be fine for a first version, but I can see the want to have something smarter)

Start by comparing the existing toolchain install and filesystem-only link command entry points. Trace how Rustup handles toolchain locations and the conventions used by Rust's CI and x.py. Done means the project has a decided interface for remote custom toolchains, installation, and the unresolved update behavior.