Allow plugins to mutate available commands sent to the client.

This is the first unstable API being introduced and is primarily to get feedback on the system.

api/build.gradle

@@ -26,6 +26,7 @@ dependencies {
     compile "org.slf4j:slf4j-api:${slf4jVersion}"
     compile 'com.google.inject:guice:4.2.2'
     compile "org.checkerframework:checker-qual:${checkerFrameworkVersion}"
+    compile 'com.mojang:brigadier:1.0.17'
 
     compile "org.spongepowered:configurate-hocon:${configurateVersion}"
     compile "org.spongepowered:configurate-yaml:${configurateVersion}"

api/src/main/java/com/velocitypowered/api/event/command/PlayerAvailableCommandsEvent.java

@@ -0,0 +1,37 @@
+package com.velocitypowered.api.event.command;
+
+import static com.google.common.base.Preconditions.checkNotNull;
+
+import com.mojang.brigadier.tree.RootCommandNode;
+import com.velocitypowered.api.annotations.UnstableApi;
+import com.velocitypowered.api.proxy.Player;
+
+/**
+ * Allows plugins to modify the packet indicating commands available on the server to a
+ * Minecraft 1.13+ client.
+ */
+@UnstableApi
+public class PlayerAvailableCommandsEvent {
+
+  private final Player player;
+  private final RootCommandNode<?> rootNode;
+
+  /**
+   * Constructs an available commands event.
+   * @param player the targeted player
+   * @param rootNode the Brigadier root node
+   */
+  public PlayerAvailableCommandsEvent(Player player,
+      RootCommandNode<?> rootNode) {
+    this.player = checkNotNull(player, "player");
+    this.rootNode = checkNotNull(rootNode, "rootNode");
+  }
+
+  public Player getPlayer() {
+    return player;
+  }
+
+  public RootCommandNode<?> getRootNode() {
+    return rootNode;
+  }
+}

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

@@ -7,6 +7,7 @@ import com.mojang.brigadier.arguments.StringArgumentType;
 import com.mojang.brigadier.builder.LiteralArgumentBuilder;
 import com.mojang.brigadier.builder.RequiredArgumentBuilder;
 import com.mojang.brigadier.tree.LiteralCommandNode;
+import com.velocitypowered.api.event.command.PlayerAvailableCommandsEvent;
 import com.velocitypowered.api.event.connection.PluginMessageEvent;
 import com.velocitypowered.api.proxy.messages.ChannelIdentifier;
 import com.velocitypowered.proxy.VelocityServer;
@@ -175,7 +176,11 @@ public class BackendPlaySessionHandler implements MinecraftSessionHandler {
           .build();
       commands.getRootNode().addChild(root);
     }
-    return false;
+
+    server.getEventManager().fire(
+        new PlayerAvailableCommandsEvent(serverConn.getPlayer(), commands.getRootNode()))
+        .thenAcceptAsync(event -> playerConnection.write(commands), playerConnection.eventLoop());
+    return true;
   }
 
   @Override