Read file gridfs into ReadStream

[liuchong]

Motivation:

Sometimes we need only a ReadStream, no need to write to a file.

[liuchong]

Please check if we need this @vietj @pmlopes @tsegismont , I will add some unit test and do more code optimization if it will merge, thanks!

[tsegismont]

Thank you @liuchong , I'll review it

[liuchong]

Ok, sorry for the delay, I will add tests in my tomorrow morning 👀

[tsegismont]

No worries, thank you!