Add SpawnReason for Entity Reinforcements. Fixes BUKKIT-4744

By: EvilSeph <evilseph@gmail.com>

paper-api/src/main/java/org/bukkit/event/entity/CreatureSpawnEvent.java

@@ -147,6 +147,10 @@ public class CreatureSpawnEvent extends EntityEvent implements Cancellable {
          * When a slime splits
          */
         SLIME_SPLIT,
+        /**
+         * When an entity calls for reinforcements
+         */
+        REINFORCEMENTS,
         /**
          * When a creature is spawned by plugins
          */