Fixes and additions to the spawn reason API

Expose an entities spawn reason on the entity.
Pre existing entities will return NATURAL if it was a non
persistenting Living Entity, SPAWNER for spawners,
or DEFAULT since data was not stored.

Additionally, add missing spawn reasons.

Co-authored-by: Aurora <aurora@relanet.eu>
Co-authored-by: Jakub Zacek <dawon@dawon.eu>

paper-api/src/main/java/org/bukkit/entity/Entity.java

@@ -857,5 +857,11 @@ public interface Entity extends Metadatable, CommandSender, Nameable, Persistent
         // TODO remove impl here
         return getLocation().getChunk();
     }
+
+    /**
+     * @return The {@link org.bukkit.event.entity.CreatureSpawnEvent.SpawnReason} that initially spawned this entity. <!-- Paper - added "initially" to clarify that the SpawnReason doesn't change after the Entity was initially spawned" -->
+     */
+    @NotNull
+    org.bukkit.event.entity.CreatureSpawnEvent.SpawnReason getEntitySpawnReason();
     // Paper end
 }