fix(types): add optional value to MultipartFile for onFile

[imharjot]

The README documents setting part.value inside an onFile handler when using
attachFieldsToBody: 'keyValues', but MultipartFile has no value property, so that
example fails to typecheck with TS2339 (Property 'value' does not exist on type 'MultipartFile').

Adds an optional value?: unknown to MultipartFile. unknown (not any) keeps it
type-safe and matches the existing Record<string, unknown> usage in this file; it's
optional because a file part only gets a value when one is assigned in onFile. Kept
MultipartFile non-generic to avoid rippling generics through Multipart/MultipartFields.

The existing type test asserted req.body.file had no value, so that assertion is flipped
and the onFile test handler now assigns part.value to cover the regression.

Fixes #574

[is2ei]

I think the issue is a bit more complicated.

It looks like assigning the value field to the part object in the onFile function is a bit of a hack.
This works because the if statement uses field.value when it is not undefined.

https://github.com/fastify/fastify-multipart/blob/main/index.js#L100-L103

            if (field.value !== undefined) {
              body[key] = field.value
            } else if (field._buf) {
              body[key] = field._buf

However, the value property is intended for objects whose type is field.

https://github.com/fastify/fastify-multipart/blob/main/index.js#L338-L347

      const value = {
        type: 'field',
        fieldname: name,
        mimetype: contentType,
        encoding,
        value: fieldValue,
        fieldnameTruncated,
        valueTruncated,
        fields: body
      }

On the other hand, the _buf property is intended for objects whose type is file.

https://github.com/fastify/fastify-multipart/blob/main/index.js#L373-L382

      const value = {
        type: 'file',
        fieldname: name,
        filename,
        encoding,
        mimetype,
        file,
        fields: body,
        _buf: null,
        async toBuffer () {