Create ActivityPub\FetchQueue and ActivityPub\FetchQueueItem classes

- These classes are used to flatten the recursive missing activity fetch that can hit PHP's maximum function nesting limit
- The original caller is responsible for processing the remaining queue once the original activity has been fetched

src/Module/Debug/ActivityPubConversion.php

@@ -123,7 +123,7 @@ class ActivityPubConversion extends BaseModule
 					'content' => visible_whitespace(var_export($object_data, true))
 				];
 
-				$item = ActivityPub\Processor::createItem($object_data);
+				$item = ActivityPub\Processor::createItem(new ActivityPub\FetchQueue(), $object_data);
 
 				$results[] = [
 					'title'   => DI::l10n()->t('Result Item'),