feat: support event.fetch and event.$fetch

[pi0]

🔗 Linked issue

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

This feature allows to use event.fetch and event.$fetch proxying context and incoming request headers easily.

export default eventHandler(async (event) => {
  const res = await event.$fetch("/echo");
  return res;
});

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.