feat: Support Multiple Files in a Single Multipart Upload

## Feature Request

### Plugin

[@capacitor/file-transfer](https://capacitorjs.com/docs/apis/file-transfer)

### Description

The startUpload function currently only accepts one file path at a time. This means that to upload files to one endpoint I have to send multiple separate requests. However, many real world backends expect a single multipart/form-data request with multiple file parts (e.g. images[0], images[1]) and have atomic logic tied to receiving them together

Can we add support for this? Both Android and iOS have built-in APIs for handling requests, with multiple file parts. So I think it should be possible to implement this feature

### Platform(s)

Android, iOS, Web

### Preferred Solution

### Alternatives

### Additional Context