feat(transport/http): remove wrapper.w and add Unwrap method for acce…

[kvii]

Description (what this PR does / why we need it):

1. Remove wrapper.w to avoid confused of ResponseWriter.
2. Add unwrap method for accessing wrapped ResponseWriter.
3. Create testing code for wrapper.Result.

Now users can use followed code to get custom ResponseWriter they passed:

if u, ok := w.(interface { Unwrap() http.ResponseWriter }); ok {
	if cw, ok := w.Unwrap().(*CustomResponseWriter); ok {
		// do something with CustomResponseWriter
	}
}

Which issue(s) this PR fixes (resolves / be part of):

fixes/resolves #3174

Other special notes for the reviewers:

Rename "responseWriter" to "wrappedWriter" for better semantics.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c7fa51d) 84.62% compared to head (33531fb) 84.60%.

❗ Current head 33531fb differs from pull request most recent head 582e3ee. Consider uploading reports for the commit 582e3ee to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3189      +/-   ##
==========================================
- Coverage   84.62%   84.60%   -0.02%     
==========================================
  Files          88       88              
  Lines        3993     3988       -5     
==========================================
- Hits         3379     3374       -5     
  Misses        440      440              
  Partials      174      174              

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