Allow non-FormFile types in multipart forms

[b-kamphorst]

I adopted Huma in my project over the last days and I really like it! In general I thought it was quite easy to adopt Huma with the exception of Enums (#621 (comment) helped me out) and multipart form data. I found the documentation on multipart form data lacking, but I could get a functioning version after looking at the tests.

My web server receives multipart form data that includes both files and strings. I made a change to Huma v2.27.0 that allows string fields within RawData/MultipartFormFiles. I am curious whether this is a meaningful addition to your work? In extention, it could be useful to allow for fields of arbitrary type within MultipartFormFiles?

Looking forward to your thoughts and keep up the good work!