Player: added a server-side forced cooldown mechanism

This is necessary because the stupid client constantly spams right-click actions if you carry on trying to eat/throw/whatever the item when cooldown is in effect. Therefore ender pearls would be fired like machine guns without these checks server side.

src/pocketmine/item/Item.php

@@ -821,6 +821,15 @@ class Item implements ItemIds, \JsonSerializable{
 		return false;
 	}
 
+	/**
+	 * Returns the number of ticks a player must wait before activating this item again.
+	 *
+	 * @return int
+	 */
+	public function getCooldownTicks() : int{
+		return 0;
+	}
+
 	/**
 	 * Compares an Item to this Item and check if they match.
 	 *