SPIGOT-6200: Add COMMAND to the CreateSpawnEvent.SpawnReason enum

By: Julian van den Berkmortel <julianvdberkmortel@outlook.com>

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

@@ -177,6 +177,10 @@ public class CreatureSpawnEvent extends EntitySpawnEvent {
          * When a piglin is converted to a zombified piglib.
          */
         PIGLIN_ZOMBIFIED,
+        /**
+         * When a creature is spawned by the "/summon" command
+         */
+        COMMAND,
         /**
          * When a creature is spawned by plugins
          */