Allow the BungeeCord plugin messaging channel to be used by the server at transition time

proxy/src/main/java/com/velocitypowered/proxy/connection/backend/TransitionSessionHandler.java

@@ -51,6 +51,7 @@ public class TransitionSessionHandler implements MinecraftSessionHandler {
   private final VelocityServer server;
   private final VelocityServerConnection serverConn;
   private final CompletableFuture<Impl> resultFuture;
+  private final BungeeCordMessageResponder bungeecordMessageResponder;
 
   /**
    * Creates the new transition handler.
@@ -64,6 +65,8 @@ public class TransitionSessionHandler implements MinecraftSessionHandler {
     this.server = server;
     this.serverConn = serverConn;
     this.resultFuture = resultFuture;
+    this.bungeecordMessageResponder = new BungeeCordMessageResponder(server,
+        serverConn.getPlayer());
   }
 
   @Override
@@ -168,6 +171,10 @@ public class TransitionSessionHandler implements MinecraftSessionHandler {
 
   @Override
   public boolean handle(PluginMessage packet) {
+    if (bungeecordMessageResponder.process(packet)) {
+      return true;
+    }
+
     if (PluginMessageUtil.isRegister(packet)) {
       serverConn.getPlayer().getKnownChannels().addAll(PluginMessageUtil.getChannels(packet));
     } else if (PluginMessageUtil.isUnregister(packet)) {