SPIGOT-826: Add method to retrieve entity by UUID

By: Zach Brown <zach.brown@destroystokyo.com>

paper-api/src/main/java/org/bukkit/Bukkit.java

@@ -1141,6 +1141,16 @@ public final class Bukkit {
         return server.createBossBar(title, color, style, flags);
     }
 
+    /**
+     * Gets an entity on the server by its UUID
+     *
+     * @param uuid the UUID of the entity
+     * @return the entity with the given UUID, or null if it isn't found
+     */
+    public static Entity getEntity(UUID uuid) {
+        return server.getEntity(uuid);
+    }
+
     /**
      * @see UnsafeValues
      * @return the unsafe values instance