bus-util: add bus_property_get_string_set() helper

This is a common implementation of a property getter for a Set of
strings.

src/shared/bus-util.c

@@ -697,3 +697,21 @@ int bus_message_append_string_set(sd_bus_message *m, Set *set) {
 
         return sd_bus_message_close_container(m);
 }
+
+int bus_property_get_string_set(
+                sd_bus *bus,
+                const char *path,
+                const char *interface,
+                const char *property,
+                sd_bus_message *reply,
+                void *userdata,
+                sd_bus_error *error) {
+
+        Set **s = ASSERT_PTR(userdata);
+
+        assert(bus);
+        assert(property);
+        assert(reply);
+
+        return bus_message_append_string_set(reply, *s);
+}