feat: NITRO_APP_BASE_URL can be used at runtime

[jvllmr]

🔗 Linked issue

closes #4381

❓ 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 is my first draft for the changes discussed in #4381
I will mark this PR as ready once I'm confident enough in the solution for further review and I added sufficient documentation.

General idea is that h3.requestWithBaseUrl is applied on the request if the config setting features.runtimeBaseUrl is set to true.

📝 Checklist

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

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✨ Finishing Touches 💡 1

🛠️ Fix failing CI checks 💡

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