Tell the backend server when a player is waiting to allow proactive freeing of slots

src/main/java/uk/co/notnull/proxyqueues/ProxyQueuesImpl.java

···
 import com.velocitypowered.api.event.ResultedEvent;
 import com.velocitypowered.api.event.Subscribe;
 import com.velocitypowered.api.event.connection.LoginEvent;
+import com.velocitypowered.api.event.connection.PluginMessageEvent;
 import com.velocitypowered.api.event.proxy.ProxyInitializeEvent;
 import com.velocitypowered.api.plugin.annotation.DataDirectory;
 import com.velocitypowered.api.proxy.ProxyServer;
+import com.velocitypowered.api.proxy.messages.MinecraftChannelIdentifier;
 import com.velocitypowered.api.proxy.server.RegisteredServer;
 import net.kyori.adventure.text.ComponentLike;
 import net.kyori.adventure.text.minimessage.MiniMessage;
···
 import java.util.function.Function;
 
 public final class ProxyQueuesImpl implements ProxyQueues {
+    public static final MinecraftChannelIdentifier playerWaitingIdentifier =
+            MinecraftChannelIdentifier.create("proxyqueues", "player_waiting");
 
     private static ProxyQueuesImpl instance;
     private final SettingsHandler settingsHandler;
···
             new ProxyDiscordHandler(this);
         }
 
+        proxyServer.getChannelRegistrar().register(playerWaitingIdentifier);
         new SuperVanishBridgeHelper(proxyServer);
     }
 
···
         if(playerLimit > -1 && proxyServer.getPlayerCount() >= playerLimit &&
                 !event.getPlayer().hasPermission("proxyqueues.bypass-limit")) {
             event.setResult(ResultedEvent.ComponentResult.denied(Messages.getComponent("errors.player-limit")));
+        }
+    }
+
+    @Subscribe
+    public void onPluginMessageFromPlayer(PluginMessageEvent event) {
+        // Don't allow spoofing of plugin messages
+        if (playerWaitingIdentifier.equals(event.getIdentifier())) {
+            event.setResult(PluginMessageEvent.ForwardResult.handled());
         }
     }
 

src/main/java/uk/co/notnull/proxyqueues/tasks/QueueMoveTask.java

···
 
         // Check if the max amount of players on the server are the max slots
         if (queue.isServerFull(targetPlayer.getQueueType())) {
+            // Tell the backend server a player is waiting to allow proactive freeing of slots
+            queue.getServer().sendPluginMessage(ProxyQueuesImpl.playerWaitingIdentifier, new byte[0]);
             return;
         }