Expose AWS GetObjectResponse when downloading files

[omerlh]

Hey! Thanks for this library - life saver! We are using the S3 file metadata to encode hints about the file, like the format - so I would like to have access to them before downloading the file as stream so I could know how to parse them. It seems that we need either to download twice - once with the pure library and once with the stream - or copy the relevant code and modify it. It will be nice if the S3 downloadMultiPart will have overload that receive the GetObjectResponse and download the file instead of getting the bucket / key