Feat: Add Http headers constants

[YlanzinhoY]

I added header constants for type safety and to avoid writing anything wrong.

I believe it's a good addition to the codebase, since competing frameworks also have this kind of thing.

This feature maintains backward compatibility, allowing those who want to write in a pure way to continue, and those who want to use constants to use them as well.

example

Please ensure your pull request meets the following requirements:

• Open your pull request against the master branch.
• All tests pass in available continuous integration systems (e.g., GitHub Actions).
• Tests are added or modified as needed to cover code changes.
• If the pull request introduces a new feature, the feature is documented in the docs/doc.md.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.99%. Comparing base (3dc1cd6) to head (85aae67).
⚠️ Report is 226 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4474      +/-   ##
==========================================
- Coverage   99.21%   98.99%   -0.22%     
==========================================
  Files          42       44       +2     
  Lines        3182     2986     -196     
==========================================
- Hits         3157     2956     -201     
- Misses         17       21       +4     
- Partials        8        9       +1     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.98% <ø> (?)
-tags go_json	98.92% <ø> (?)
-tags nomsgpack	98.98% <ø> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.24	98.99% <ø> (?)
go-1.25	98.99% <ø> (?)
macos-latest	98.99% <ø> (-0.22%)	⬇️
ubuntu-latest	98.99% <ø> (-0.22%)	⬇️

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[anfaas1618]

can we not make a type here named

type Header string

and use

const (
       HeaderAuthorization Header = "Authorization"
       ...
)

[anfaas1618]

link this to a issue please