feat(transport/http): add unwrap method for returning underlying response writer

[kvii]

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

Add an Unwrap method for returning underlying response writer.

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

resolves #3253

Other special notes for the reviewers:

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.64%. Comparing base (1fdaabb) to head (1d9b5a4).

❗ Current head 1d9b5a4 differs from pull request most recent head 7fc0bc5. Consider uploading reports for the commit 7fc0bc5 to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3265      +/-   ##
==========================================
+ Coverage   81.59%   81.64%   +0.05%     
==========================================
  Files          91       91              
  Lines        4167     4168       +1     
==========================================
+ Hits         3400     3403       +3     
+ Misses        589      587       -2     
  Partials      178      178              

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