bind : add support of multipart multi files

[martinyonatann]

Summary
This PR introduces support for handling multipart requests that contain multiple files in the bind function. It extends the current functionality to allow seamless parsing and binding of multiple files uploaded through multipart form data.

Changes

• Modified the bind logic to handle multiple file uploads in a single request.
• Added support for accessing multiple files via FormFile.
• Updated the relevant documentation and comments for clarity.
• Introduced test cases to validate multi-file uploads.

Testing

• Added unit tests to ensure multi-file binding is working as expected.

Additional Information
This update improves the flexibility of the bind function when dealing with file uploads, making it easier to handle bulk file operations in a single request.

[martinyonatann]

to resolve this issue #2672

[martinyonatann]

any update @aldas ?

[aldas]

This is a good improvement. Thank you.

I took some liberties and modified your PR to my maintainer preferences aldas@8c702f4

I think plain/non-pointer multipart.FileHeader should not be allowed bound to, as you can not determine easily from struct field if it was bound into or not. Field of type multipart.FileHeader struct sound more like field that MUST be bound to but Echo struct tag binding does not have concept of "must". Does this sound acceptable?

If you agree to, I would edit this PR.

[martinyonatann]

hi @aldas, i agree with your suggestions. Please feel free to proceed with editing the PR as you see fit.