added callback on every read attempt during multipart upload

Usefull when you need to add progress bar for upload process.


```ruby
require 'progressbar'

#...

progressbar = ProgressBar.new 'Uploading', file.size
progressbar.file_transfer_mode
key.put_multipart file do |part_data|
  progressbar.inc part_data.size
end
progressbar.finish
```