fix(static): only add vary response header when serving public assets if compression is enabled

[paro-paro]

🔗 Linked issue

#3501

❓ Type of change

• 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• 🐞 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 PR aims to fix the current implementation of the vary response header from the nitro server.

Right now, every request that includes the accept-encoding header will get back a response with vary: accept-encoding despite nitro not serving different content. This should only happen when nitro serves different compressed (on build time) versions of public assets (when the compressPublicAssets config option is enabled).

This PR fixes that and also handles this other related issue #3077, where the vary header is wrongly omitted if the encoding request header is not sent (or if set to a different value than the ones supported by EncodingMap) for compressed assets.

📝 Checklist

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

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