fix(nuxt): proxy headers to islands + returned prerender hints

[danielroe]

🔗 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)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

When using server components, currently they are unable to hint to Nitro to prerender resources. For example, a <nuxt-img> rendered in a server component will not output a statically generated image at the end of the build.

I think it makes sense to proxy both event context (e.g. auth headers) to islands as well as to set any headers on the response. (Honestly, I thought event fetch did the latter piece anyway, but it seems I am mistaken: cc: @pi0.)

📝 Checklist

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

[huang-julien]

I even thought that all headers were and should be proxied