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

By: T00thpick1 <t00thpick1dirko@gmail.com>

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);
 }