creds: Add ImportCredential=

ImportCredential= takes a credential name and searches for a matching
credential in all the credential stores we know about it. It supports
globs which are expanded so that all matching credentials are loaded.

src/core/execute.h

@@ -361,6 +361,7 @@ struct ExecContext {
 
         Hashmap *set_credentials; /* output id → ExecSetCredential */
         Hashmap *load_credentials; /* output id → ExecLoadCredential */
+        Set *import_credentials;
 
         ImagePolicy *root_image_policy, *mount_image_policy, *extension_image_policy;
 };