I noticed that the extension mechanism for guix-service-type don't 
expose authorized keys and substitute-urls. I want to create a service 
to easily set up new channels with their associated substitutes server, 
but I have to go through modify-services.

I realize that this change isn't trivial as it breaks some people's 
os.scm and requires updating the documentation, but I think it is a very 
low hanging fruit in terms of functionality.

Maybe there is a reason why things are the way they are, I would greatly 
appreciate comments.

Cheers.