chore(http): add support for go1.20 http.rwUnwrapper to gin.responseWriter

[davidmdm]

This PR implements the go1.20 http.rwUnwrapper interface on the private gin.responseWriter type:

type rwUnwrapper interface {
	Unwrap() ResponseWriter
}

This will allow code using go1.20 to use the http.ResponseController utility type added in go1.20.

engine.GET("/path", func (c *gin.Context) {
  controller := http.NewResponseController(c.Writer)
  controller.SetWriteDeadline(time.Second)
  // ...
}

[codecov]

Codecov Report

Merging #3489 (7019960) into master (c5fd063) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3489   +/-   ##
=======================================
  Coverage   98.63%   98.63%           
=======================================
  Files          42       42           
  Lines        3140     3142    +2     
=======================================
+ Hits         3097     3099    +2     
  Misses         29       29           
  Partials       14       14           

Flag	Coverage Δ
	98.63% <100.00%> (+<0.01%)	⬆️
go-1.16	∅ <ø> (?)
go-1.17	98.53% <100.00%> (+<0.01%)	⬆️
go-1.18	98.53% <100.00%> (+<0.01%)	⬆️
go-1.19	98.63% <100.00%> (+<0.01%)	⬆️
macos-latest	98.63% <100.00%> (+0.09%)	⬆️
ubuntu-latest	98.63% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
response_writer.go	93.87% <100.00%> (+0.26%)	⬆️

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[appleboy]

@thinkerou help to review.