services: cgit: Make project-list permit a file-object.

Instead of having the service manage the list, it's useful to be able to point
this at an existing file, for example, when using cgit together with gitolite.

* gnu/services/cgit.scm (project-list?): New procedure.
(serialize-project-list): Handle file-object values.
(<cgit-configuration>): Change the predicate for project-list to allow lists
and file-objects.