Fix empty request body on retries with compression enabled

[merlinz01]

Description

Make the transport use a new reader for each retry when gzip compression is enabled.
Also be more explicit about doing so for non-compressed requests.
Test included.

Issues Resolved

Closes #541.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.96%. Comparing base (06a6dc8) to head (5258878).
Report is 22 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #542       +/-   ##
===========================================
+ Coverage   57.29%   67.96%   +10.67%     
===========================================
  Files         315      376       +61     
  Lines        9823     8853      -970     
===========================================
+ Hits         5628     6017      +389     
+ Misses       2902     1555     -1347     
+ Partials     1293     1281       -12     

Flag	Coverage Δ
integration	60.39% <100.00%> (+9.55%)	⬆️
unit	15.28% <100.00%> (+2.43%)	⬆️

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

Files	Coverage Δ
opensearchtransport/opensearchtransport.go	86.63% <100.00%> (+1.47%)	⬆️

... and 272 files with indirect coverage changes

[merlinz01]

I forgot to create a new branch for the PR. Please disregard.