feat(nuxt): support app config for server routes

[pi0]

🔗 Linked issue

#14670 via unjs/nitro#1022

❓ 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

This feature supports useAppConfig for nitro server routes and seamless integration with nuxt (public) app.config.ts in project's root level. In addition to this nitro also supports server/app.config.ts for private app config.

(pending nitropack release made PR early to preview)

Next steps

• Bundle app config only once to the server. Currently it bundles to both vite server and nitro server while nitro could expose it for vite on (production build) for server-side.
• Allow to explicitly provide private app.config, (we might think about a new server.config/serverConfig however there might be better alternatives we could think later)

📝 Checklist

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

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