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/core/execute.h

@@ -273,6 +273,7 @@ struct ExecContext {
         size_t n_mount_images;
         MountImage *extension_images;
         size_t n_extension_images;
+        char **extension_directories;
 
         uint64_t capability_bounding_set;
         uint64_t capability_ambient_set;