fix(#128): add extensions to ReqwestMiddleware::parts

[JakubKoralewski]

Fixes #128

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.87%. Comparing base (1ff54c2) to head (b1c3fb4).
⚠️ Report is 57 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #129      +/-   ##
==========================================
- Coverage   93.91%   85.87%   -8.05%     
==========================================
  Files          10       19       +9     
  Lines        1233     7502    +6269     
==========================================
+ Hits         1158     6442    +5284     
- Misses         75     1060     +985     

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