Added downloadAsStream method to StorageObject

[gedimin45]

This let's retrieve the body of the StorageObject as a stream. Inspired by PSR-7 response method getBody. This method returns a PSR-7 StreamInterface which enables the user to decide whether they want to write stream to another stream or get the contents as a string.
I also propose to remove downloadAsString and downloadToFile in the next major release since the functionality of these methods is easily achieved using getBody and having the other methods can be confusing.

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[gedimin45]

I signed it!

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CLAs look good, thanks!

[dwsupplee]

I like the thought of exposing the streams, but am concerned getBody might be too unfamiliar for those who don't have intimate knowledge of the concepts found in PSR-7. Also, I like keeping the helper methods such as downloadToFile as depending on the use case it can save some keystrokes for a user and means they won't need to investigate how to perform that action themselves.

@Ged15 What are your thoughts on getBody => downloadStream / downloadAsStream?

[gedimin45]

downloadAsStream also sounds pretty clear 😄 shall I rename it?

[dwsupplee]

Please! :) Thanks for the quick response.

[gedimin45]

@dwsupplee I also reused the downloadAsStream method in other methods to avoid duplication

[dwsupplee]

This looks awesome. Thanks again @Ged15!