monitor: Keep track of the for lease status

Add a flag to MetaMonitor indicating if the monitor is available for
lease and store/update it from the monitor configuration.

Also, add unit tests validating that the configuration is applied and
that invalid configurations fail.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4112>

src/backends/meta-monitor.h

@@ -348,3 +348,9 @@ void meta_monitor_set_backlight (MetaMonitor *monitor,
 META_EXPORT_TEST
 gboolean meta_monitor_get_backlight (MetaMonitor *monitor,
                                      int         *value);
+
+void meta_monitor_set_for_lease (MetaMonitor *monitor,
+                                 gboolean     for_lease);
+
+META_EXPORT_TEST
+gboolean meta_monitor_is_for_lease (MetaMonitor *monitor);