MDL-32235 introducing new workshop scheduled allocation plugin

The idea is to build a plugin that will internally call the random
allocator when the submissions deadline is reached. The teacher is able
to pre-define the configuration for the random allocator.

lib/pluginlib.php

@@ -516,7 +516,7 @@ class plugin_manager {
             ),
 
             'workshopallocation' => array(
-                'manual', 'random'
+                'manual', 'random', 'scheduled'
             ),
 
             'workshopeval' => array(