service extensions to guix-service-type

[Justin Veilleux <terramorpha@cock.li>]

Hi guix-devel.

For my os.scm, I want to create a service which encapsulates the process 
of adding channels to the global channels file, adding substitute urls 
to the daemon and also adding authorized keys to the daemon. With the 
service extension system, I was able to do the first thing by extending 
`special-files-service-type`. However, I cannot do the last two things, 
because the current definition of guix-service-type only exposes 
chroot-directories (what is the use case for this, btw?).

Is there a reason for this particular design? I will probably submit a 
patch to add this functionality.

Cheers.