feat: allow arbitrary fields in MultipartFormFiles

[b-kamphorst]

@danielgtaylor I started working on #694. The first part of the work is a major refactoring that I submitted separately in #705. The second part is a feature where MultipartFormFiles also parses and validates multipart.Form.Value into the input struct. The diff is probably clearer in this PR.

At time of writing, the added feature only parses fields that have a form tag, mostly because I copied the approach for parsing and validating other request parameters and I was not quite sure how to do that differently. Before I continue, I would much like to hear your thoughts:

1. Would you consider this feature for Huma?
2. If so, do you have any feedback on how to continue with this PR?

Closes #694.

[codecov]

Codecov Report

Attention: Patch coverage is 92.22222% with 7 lines in your changes missing coverage. Please review.

Project coverage is 92.84%. Comparing base (1c3924e) to head (c71c254).

Files with missing lines	Patch %	Lines
huma.go	90.90%	4 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #706      +/-   ##
==========================================
- Coverage   92.90%   92.84%   -0.07%     
==========================================
  Files          22       22              
  Lines        4963     5014      +51     
==========================================
+ Hits         4611     4655      +44     
- Misses        305      309       +4     
- Partials       47       50       +3     

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[Copilot]

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

Comments suppressed due to low confidence (2)

formdata.go:125

• Potential nil pointer dereference. Check if formValueParser is nil before calling it.

formValueParser(value)

formdata.go:178

• [nitpick] Error message 'Failed to open file' is not descriptive. Consider including more context about the failure.

return FormFile{}, &ErrorDetail{Message: "Failed to open file", Location: location}

[danielgtaylor]

@b-kamphorst I think this looks good. Do you mind adding some additional user-facing documentation for form handling and the new feature? Thanks!

[b-kamphorst]

Thank you for the initial review. I will update on the documentation shortly.

Question: is it acceptable that only struct fields with a form tag are passed? This deviaties from the parsing of non-form request data, but I did it so that I could easily (/ knew how to) reuse validation logic.

[b-kamphorst]

Oh and I appreciate your merging effort :)

[b-kamphorst]

@danielgtaylor I think this extention to the docs suffices, but please let me know what you think.