bug#50606: Add support for other formats of Guix channels

[zimoun]

Hi,

On Fri, 17 Sept 2021 at 12:47, EuAndreh <e...@euandre.org> wrote:
>
> Shall we close this bug?

I think this wish,

Guix channel authors may want to serve channels:
- via "Dumb HTTP" Git repositories;

still makes sense.

Cheers,
simon