Support custom filenames

[erickjth]

Summary

The goal of the update is to allow using a custom file name in the upload request. Sometimes it is useful to use a different name from the original file name.

This update includes some logic in the native code (android/iOS) to use a custom file name at the time of creating the request.

Also, it includes a new setting in the uploader { customFilename: string }. This PR also updates the README file.

Test Plan

What's required for testing (prerequisites)?

Just include the customFilename into the options.

const options = {
  url: 'https://myservice.com/path/to/post',
  path: 'file://path/to/file%20on%20device.png',
  method: 'POST',
  field: 'uploaded_media',
  customFilename: 'new_filename.png',
  type: 'multipart'
}

Compatibility

OS	Implemented
iOS	✅
Android	✅