services: guix: Add 'authorized-keys' configuration knob.

* gnu/services/base.scm (hydra-key-authorization): Add 'key' parameter
and honor it.
(%default-authorized-guix-keys): New variable.
(<guix-configuration>)[authorized-keys]: New field.
(guix-shepherd-service): Adjust 'match' clause accordingly.
(guix-activation): Adjust call to 'hydra-key-authorization'.
* doc/guix.texi (Base Services): Document 'authorized-keys'.