SPIGOT-826: Add method to retrieve entity by UUID

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

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

@@ -935,6 +935,14 @@ public interface Server extends PluginMessageRecipient {
      */
     BossBar createBossBar(String title, BarColor color, BarStyle style, BarFlag ...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
+     */
+    Entity getEntity(UUID uuid);
+
     /**
      * @see UnsafeValues
      * @return the unsafe values instance