feat: add multipart file upload streaming #309

[jeevatkm]

This PR adds the long-awaited feature of multipart file upload streaming in Resty. There are two behavior -

• If any file or io. Reader is detected in the MultipartField; Resty, by default, will stream the content in the request body
• Otherwise, Resty will do simple multipart form data gets composed in memory to fire the request

This PR opens the way for the next feature, i.e., multipart live upload progress callback.

closes #309
closes #495
closes #576
closes #650

[codecov]

Codecov Report

Attention: Patch coverage is 90.26549% with 11 lines in your changes missing coverage. Please review.

Project coverage is 98.35%. Comparing base (c6cf259) to head (2e127ca).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
middleware.go	81.25%	4 Missing and 5 partials ⚠️
request.go	92.59%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #879      +/-   ##
==========================================
+ Coverage   98.15%   98.35%   +0.20%     
==========================================
  Files          14       15       +1     
  Lines        2329     2431     +102     
==========================================
+ Hits         2286     2391     +105     
+ Misses         23       21       -2     
+ Partials       20       19       -1     

Flag	Coverage Δ
unittests	98.35% <90.26%> (+0.20%)	⬆️

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