Fix commands from signs not firing command events

This patch changes sign command logic so that `run_command` click events:
  - are logged to the console
  - fire PlayerCommandPreprocessEvent
  - work with double-slash commands like `//wand`
  - sends failure messages to the player who clicked the sign

paper-server/src/main/java/io/papermc/paper/commands/DelegatingCommandSource.java

@@ -0,0 +1,42 @@
+package io.papermc.paper.commands;
+
+import net.minecraft.commands.CommandSource;
+import net.minecraft.commands.CommandSourceStack;
+import net.minecraft.network.chat.Component;
+import org.bukkit.command.CommandSender;
+
+import java.util.UUID;
+
+public class DelegatingCommandSource implements CommandSource {
+
+    private final CommandSource delegate;
+
+    public DelegatingCommandSource(CommandSource delegate) {
+        this.delegate = delegate;
+    }
+
+    @Override
+    public void sendSystemMessage(Component message) {
+        delegate.sendSystemMessage(message);
+    }
+
+    @Override
+    public boolean acceptsSuccess() {
+        return delegate.acceptsSuccess();
+    }
+
+    @Override
+    public boolean acceptsFailure() {
+        return delegate.acceptsFailure();
+    }
+
+    @Override
+    public boolean shouldInformAdmins() {
+        return delegate.shouldInformAdmins();
+    }
+
+    @Override
+    public CommandSender getBukkitSender(CommandSourceStack wrapper) {
+        return delegate.getBukkitSender(wrapper);
+    }
+}