fix: formData handling on Vercel

[JarrodMFlesch]

Description

Adjust how body data is read based on the size.

IF the content-type is multipart:

• If the body is <= 4MB, read the body with req.formData()
• Else use busboy to read the body stream

Busboy does not seem to function properly on Vercel. (May be incompatible, searching online lead to inconclusive results, I assume it has to do with calling request.body.getReader()).

• I have read and understand the CONTRIBUTING.md document in this repository.

Type of change

• Chore (non-breaking change which does not add functionality)

Checklist:

• I have added tests that prove my fix is effective or that my feature works
• Existing test suite passes locally with my changes
• I have made corresponding changes to the documentation