feat(gen): add size to MultipartFile struct

[ldmonster]

If we parse Multipart File, we have a size in parsing result.
I think, we can send file size to user in multipart file struct, to use next.

Purpose:
When i work with min.io, i'm stuck with storage my object, because it want's file size to use different store algorithms.
So, i made this fix for myself.

Generators: OK
Tests: OK

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.87%. Comparing base (dfdcdd7) to head (fc23362).
Report is 1 commits behind head on main.

❗ Current head fc23362 differs from pull request most recent head 06b65f9

Please upload reports for the commit 06b65f9 to get more accurate results.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1247   +/-   ##
=======================================
  Coverage   73.87%   73.87%           
=======================================
  Files         189      189           
  Lines       12772    12772           
=======================================
  Hits         9435     9435           
  Misses       2796     2796           
  Partials      541      541           

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[tdakkota]

Please change the commit message to conform ogen contributing guide.

Generated code changes should be in a separate commit.

Thanks.