Add leash API. Adds BUKKIT-4459 and BUKKIT-4583

By: T00thpick1 <t00thpick1dirko@gmail.com>

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

@@ -26,6 +26,10 @@ public enum EntityType {
      * An experience orb.
      */
     EXPERIENCE_ORB("XPOrb", ExperienceOrb.class, 2),
+    /**
+     * A leash attached to a fencepost.
+     */
+    LEASH_HITCH("LeashKnot", LeashHitch.class, 8),
     /**
      * A painting on a wall.
      */

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

@@ -0,0 +1,7 @@
+package org.bukkit.entity;
+
+/**
+ * Represents a Leash Hitch on a fence
+ */
+public interface LeashHitch extends Hanging {
+}

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

@@ -369,4 +369,31 @@ public interface LivingEntity extends Entity, Damageable {
      * @return if the custom name is displayed
      */
     public boolean isCustomNameVisible();
+
+    /**
+     * Returns whether the entity is currently leashed.
+     *
+     * @return whether the entity is leashed
+     */
+    public boolean isLeashed();
+
+    /**
+     * Gets the entity that is currently leading this entity.
+     *
+     * @return the entity holding the leash
+     * @throws IllegalStateException if not currently leashed
+     */
+    public Entity getLeashHolder() throws IllegalStateException;
+
+    /**
+     * Sets the leash on this entity to be held by the supplied entity.
+     * <p>
+     * This method has no effect on EnderDragons, Withers, Players, or Bats.
+     * Non-living entities excluding leashes will not persist as leash
+     * holders.
+     *
+     * @param holder the entity to leash this entity to
+     * @return whether the operation was successful
+     */
+    public boolean setLeashHolder(Entity holder);
 }