Add responseHeaders to the data object when upload is completed

[siiiiilvio]

Summary

When using your this library with your own multipart uploader, you may need some additional information from the response such as Etag headers from Amazon S3.

Test Plan

When the response is valid, populate the responseHeaders with the response headers using allHeaderFields.

What's required for testing (prerequisites)?

Test completed, cancelled and error scenarios.

What are the steps to reproduce (after prerequisites)?

When upload is completed, inspect the data object with the nested responseHeaders object.

Compatibility

| iOS | ✅
| Android | ✅ (Already implemented by https://github.com/Vydia/react-native-background-upload/pull/265)

Checklist

• [X] I have tested this on a device and a simulator
• [X] I added the documentation in README.md
• [X] I updated the typed files (TS and Flow)
• [ ] I've added Detox End-to-End Test(s)
• [ ] I've created a snack to demonstrate the changes: LINK HERE

[jkerb8]

Any chance this gets merged? Needed for multipart uploads to an S3 bucket on iOS. Seems to already work on android.