SPIGOT-6918: Add SpawnCategory API and configurations for Axolotls

By: Doc <nachito94@msn.com>

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

@@ -638,4 +638,12 @@ public interface Entity extends Metadatable, CommandSender, Nameable, Persistent
      */
     @NotNull
     Pose getPose();
+
+    /**
+     * Get the category of spawn to which this entity belongs.
+     *
+     * @return the entity´s category spawn
+     */
+    @NotNull
+    SpawnCategory getSpawnCategory();
 }

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

@@ -0,0 +1,43 @@
+package org.bukkit.entity;
+
+/**
+ * Represents groups of entities with shared spawn behaviors and mob caps.
+ *
+ * @see <a href="https://minecraft.fandom.com/wiki/Spawn#Java_Edition_mob_cap">Minecraft Wiki</a>
+ */
+public enum SpawnCategory {
+
+    /**
+     * Entities related to Monsters, eg: Witch, Zombie, Creeper, etc.
+     */
+    MONSTER,
+    /**
+     * Entities related to Animals, eg: Strider, Cow, Turtle, etc.
+     */
+    ANIMAL,
+    /**
+     * Entities related to Water Animals, eg: Squid or Dolphin.
+     */
+    WATER_ANIMAL,
+    /**
+     * Entities related to Water Ambient, eg: Cod, PufferFish, Tropical Fish,
+     * Salmon, etc.
+     */
+    WATER_AMBIENT,
+    /**
+     * Entities related to Water Underground, eg: Glow Squid.
+     */
+    WATER_UNDERGROUND_CREATURE,
+    /**
+     * Entities related to Ambient, eg: Bat.
+     */
+    AMBIENT,
+    /**
+     * All the Axolotl are represented by this Category.
+     */
+    AXOLOTL,
+    /**
+     * Entities not related to a mob, eg: Player, ArmorStand, Boat, etc.
+     */
+    MISC;
+}