maintainers-list: create a field inactivityReason

Default null; should be a free-form string explaining why the maintainer
became inactive.

The most immediate use of this attribute is to mark maintainers that are
not contributing to Nixpkgs from a long amount of time. Nonetheless it can
be used to mark any *long term inactivity*.

lib/tests/maintainer-module.nix

@@ -28,5 +28,9 @@ in {
       });
       default = [];
     };
+    inactivityReason = lib.mkOption {
+      type = types.nullOr types.str;
+      default = null;
+    };
   };
 }