feat: preserve prerendered asset content-type header

[danielroe]

🔗 Linked issue

resolves #1586

❓ 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 preserves content-type header and allows user overrides. We only store metadata if the content type differs from the one we would infer in the public-assets plugin.

We might in future support inferring more things, perhaps caching headers?

📝 Checklist

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

[codecov]

Codecov Report

Merging #1587 (5de0d06) into main (a90a1b9) will decrease coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1587      +/-   ##
==========================================
- Coverage   76.23%   76.17%   -0.06%     
==========================================
  Files          76       76              
  Lines        7788     7799      +11     
  Branches      777      778       +1     
==========================================
+ Hits         5937     5941       +4     
- Misses       1849     1855       +6     
- Partials        2        3       +1     

Files Changed	Coverage Δ
src/prerender.ts	86.05% <100.00%> (+0.30%)	⬆️
src/rollup/plugins/public-assets.ts	92.68% <100.00%> (ø)
src/types/nitro.ts	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes