Add callback for client.Upload/client.Download options

[fheinecke]

Objective

To be able to modify arbitrary upload/download options, such as adding metadata to uploads or downloading specific versions. Currently there are callbacks to change client options (i.e. downloaderOpts, uploaderOpts), but these do not provide the functionality I'm requesting. Adding support for callbacks that are provided s3manager.UploadInput before it is passed to s3manager.Uploader::Upload would greatly increase the flexibility of the tool.

Deliverables

• Support modifying s3manager.UploadInput struct before upload
• Support modifying s3.GetObjectInput struct before download