…rm data handling

The PostMultipart function has been modified to accept a slice of FormField structs instead of a map, enhancing the structure and flexibility of multipart requests. The createMultipartReader function has been adjusted to support this change.

Implemented tests for the Collector's PostMultipart method, covering basic multipart form submissions, file uploads, and submissions with different fields. Enhanced the test server to handle multipart form data correctly, ensuring robust validation of response status codes.