core: add ExtensionDirectories= setting

Add a new setting that follows the same principle and implementation
as ExtensionImages, but using directories as sources.
It will be used to implement support for extending portable images
with directories, since portable services can already use a directory
as root.

src/shared/bus-unit-util.c

@@ -973,6 +973,7 @@ static int bus_append_execute_property(sd_bus_message *m, const char *field, con
                               "ExecPaths",
                               "NoExecPaths",
                               "ExecSearchPath",
+                              "ExtensionDirectories",
                               "ConfigurationDirectory",
                               "SupplementaryGroups",
                               "SystemCallArchitectures"))