Make the registry implement the download protocol

[arschles]

I said I would do this a while ago, and then it slipped my mind 😦

@marwan-at-work I know you had interest in doing this? Let me know, and zero problems if not - I'll take care of it in that case.

[marwan-at-work]

will take a stab at it : )

[marwan-at-work]

Fixed by by #238

However, the one endpoint that remains unimplemented by both the registry and the proxy, is the /@latest endpoint which is pending input from the Go team via golang/go#26334

Since we already have an issue for that endpoint (#243)
I think it's okay to close this issue

[ghost]

thanks @marwan-at-work