feat: allow custom global options for `$fetch.create`

[CafuChino]

🔗 Linked issue

71

❓ 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)
• [x] ✨ 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 commit adds an optional parameter globalOptionsOverwrite to $fetch.create.

However, for HTTP, headers are not case-sensitive. But for some reason, headers are case-sensitive in some old backend frameworks. This optional parameter overrides native Headers behaviour in such situations.

As only adding an optional parameter, it won`t break anything, maybe?

📝 Checklist

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 67.22%. Comparing base (27996d3) to head (8ff591b). Report is 36 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #401 +/- ## =========================================== + Coverage 56.86% 67.22% +10.35% =========================================== Files 16 16 Lines 728 476 -252 Branches 113 116 +3 =========================================== - Hits 414 320 -94 + Misses 303 146 -157 + Partials 11 10 -1 ```

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