Added new methods to Server, Player and OfflinePlayer for banning and unbanning

By: Dinnerbone <dinnerbone@dinnerbone.com>

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

@@ -407,4 +407,11 @@ public interface Server {
      * @param address IP address to unban
      */
     public void unbanIP(String address);
+
+    /**
+     * Gets a set containing all banned players
+     *
+     * @return Set containing banned players
+     */
+    public Set<OfflinePlayer> getBannedPlayers();
 }