Add `get` methods to the server hook

src/main/java/emu/grasscutter/plugin/api/ServerHook.java

 package emu.grasscutter.plugin.api;
 
+import emu.grasscutter.command.Command;
+import emu.grasscutter.command.CommandHandler;
 import emu.grasscutter.game.GenshinPlayer;
 import emu.grasscutter.server.dispatch.DispatchServer;
 import emu.grasscutter.server.game.GameServer;
@@ -35,6 +37,20 @@ public final class ServerHook {
         instance = this;
     }
 
+    /**
+     * @return The game server.
+     */
+    public GameServer getGameServer() {
+        return this.gameServer;
+    }
+
+    /**
+     * @return The dispatch server.
+     */
+    public DispatchServer getDispatchServer() {
+        return this.dispatchServer;
+    }
+    
     /**
      * Gets all online players.
      * @return Players connected to the server.
@@ -42,4 +58,16 @@ public final class ServerHook {
     public List<GenshinPlayer> getOnlinePlayers() {
         return new LinkedList<>(this.gameServer.getPlayers().values());
     }
+
+    /**
+     * Registers a command to the {@link emu.grasscutter.command.CommandMap}.
+     * @param handler The command handler.
+     */
+    public void registerCommand(CommandHandler handler) {
+        Class<? extends CommandHandler> clazz = handler.getClass();
+        if(!clazz.isAnnotationPresent(Command.class))
+            throw new IllegalArgumentException("Command handler must be annotated with @Command.");
+        Command commandData = clazz.getAnnotation(Command.class);
+        this.gameServer.getCommandMap().registerCommand(commandData.label(), handler);
+    }
 }
\ No newline at end of file