MDL-75256 task: move abstract get_name method to base class.

Allow adhoc tasks to implement this method, so they too can have
descriptive names for themselves. Default implementation added to
return the class name itself.

lib/classes/task/task_base.php

@@ -62,6 +62,13 @@ abstract class task_base {
     /** @var int $pid - PHP process ID that is running the task */
     private $pid = null;
 
+    /**
+     * Get a descriptive name for the task (shown to admins)
+     *
+     * @return string
+     */
+    abstract public function get_name();
+
     /**
      * Set the current lock for this task.
      * @param \core\lock\lock $lock