chore(response_writer): add Unwrap() method to ResponseWriter interface

[raju-mechatronics]

Description

This PR adds the Unwrap() http.ResponseWriter method to the exported ResponseWriter interface.

Motivation

• The concrete *responseWriter type already implements Unwrap().
• However the interface does not declare it.
• This creates an inconsistency: code that receives a gin.ResponseWriter cannot safely/portably call .Unwrap().
• Passing the underlying http.ResponseWriter to third-party handlers/middleware (c.Writer.Unwrap())

Adding it to the interface makes usage more idiomatic and type-safe.

// returns underlying http.ResponseWriter
responseWriter := c.Writer.Unwrap()

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.00%. Comparing base (3dc1cd6) to head (1fde324).
⚠️ Report is 238 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4506      +/-   ##
==========================================
- Coverage   99.21%   99.00%   -0.22%     
==========================================
  Files          42       44       +2     
  Lines        3182     3012     -170     
==========================================
- Hits         3157     2982     -175     
- Misses         17       21       +4     
- Partials        8        9       +1     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.99% <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	99.00% <ø> (?)
go-1.25	99.00% <ø> (?)
macos-latest	99.00% <ø> (-0.22%)	⬇️
ubuntu-latest	99.00% <ø> (-0.22%)	⬇️

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[raju-mechatronics]

@appleboy is this useful? The unwrap function is already implemented. I just exposed it via the ResponseWriter interface.