MDL-57442 core_calendar: Add missing method to times interface

Also adds a missing member to event_times implementation.

Part of MDL-55611 epic.

calendar/classes/local/event/value_objects/event_times.php

@@ -50,6 +50,11 @@ class event_times implements times_interface {
      */
     protected $sort;
 
+    /**
+     * @var \DateTimeImmutable $modified Time event was last modified.
+     */
+    protected $modified;
+
     /**
      * Constructor.
      *

calendar/classes/local/interfaces/times_interface.php

@@ -60,4 +60,11 @@ interface times_interface {
      * @return \DateTimeImmutable
      */
     public function get_sort_time();
+
+    /**
+     * Get the modified time.
+     *
+     * @return \DateTimeImmutable
+     */
+    public function get_modified_time();
 }