fix(nuxt): only fetch payloads for prerendered routes

[danielroe]

🔗 Linked issue

#14507

resolves #15024

❓ 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 PR adds support for an app manifest (at /manifest.json) which contains, at the moment, simply a list of routes for which there are corresponding payloads. With the implementation of route rules this manifest can be extended/modified. In addition, it can be extended with a build hash to allow version change detection.

With this PR:

• we will not fetch payloads for pages that were not prerendered (they would give a 404)
• will will fetch all payloads if there is an SSR/hybrid app
• we can fetch payloads in development mode if experimental.payloadExtraction is set by the user to true (there are more edge cases, such as CORS, with client-side fetching, so I felt it was better to set default to false in dev mode so user handles them properly, in case they are not prerendering all their pages).
• support for hybrid payload extraction (e.g. some but not all routes have SSR payloads) is deferred to implementation of route rules

(Note, @pi0, I'm also happy to go ahead and implement route rules, but thought we could adopt this approach for now and modify it at that point.)

📝 Checklist

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

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