core: add RootHash and RootVerity service parameters

Allow to explicitly pass root hash (explicitly or as a file) and verity
device/file as unit options. Take precedence over implicit checks.

src/nspawn/nspawn.c

@@ -5141,7 +5141,7 @@ static int run(int argc, char *argv[]) {
                                 goto finish;
                         }
 
-                        r = verity_metadata_load(arg_image, arg_root_hash ? NULL : &arg_root_hash, &arg_root_hash_size,
+                        r = verity_metadata_load(arg_image, NULL, arg_root_hash ? NULL : &arg_root_hash, &arg_root_hash_size,
                                         arg_verity_data ? NULL : &arg_verity_data);
                         if (r < 0) {
                                 log_error_errno(r, "Failed to read verity artefacts for %s: %m", arg_image);