Support in-memory data for Transfer Manager download_chunks_concurrently and upload_chunks_concurrently

[MattIrv]

A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]

• Today Transfer Manager's download_chunks_concurrently and upload_chunks_concurrently require you to provide a filename. Unfortunately I have bytes that are already in-memory and may not fit on disk (or it may be slow to persist it to disk) and can't use Transfer Manager as a result.

A clear and concise description of what you want to happen.

• Provide functions that support the same interface as download_chunks_concurrently and upload_chunks_concurrently but take a file-like object instead of a filename
• Or, for write, provide a similar function that returns a stream to the caller, and then writes to the stream are uploaded via multipart upload as needed (not sure if there's any realistic similar thing here for reads)
• It's ok if this functionality can only be threaded and not use multi-processing since in practice we still see a large speed increase from threading

[MattIrv]

@andrewsg fyi