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

By: T00thpick1 <t00thpick1dirko@gmail.com>

paper-api/src/main/java/org/bukkit/event/player/PlayerUnleashEntityEvent.java

@@ -0,0 +1,36 @@
+package org.bukkit.event.player;
+
+import org.bukkit.entity.Entity;
+import org.bukkit.entity.Player;
+import org.bukkit.event.Cancellable;
+import org.bukkit.event.entity.EntityUnleashEvent;
+
+/**
+ * Called prior to an entity being unleashed due to a player's action.
+ */
+public class PlayerUnleashEntityEvent extends EntityUnleashEvent implements Cancellable {
+    private final Player player;
+    private boolean cancelled = false;
+
+    public PlayerUnleashEntityEvent(Entity entity, Player player) {
+        super(entity, UnleashReason.PLAYER_UNLEASH);
+        this.player = player;
+    }
+
+    /**
+     * Returns the player who is unleashing the entity.
+     *
+     * @return The player
+     */
+    public Player getPlayer() {
+        return player;
+    }
+
+    public boolean isCancelled() {
+        return cancelled;
+    }
+
+    public void setCancelled(boolean cancel) {
+        this.cancelled = cancel;
+    }
+}