The way Steam works does not require you to do any authentication, you simply communicate thru Steam library and if user is logged it will use that user account.
UE4 has got system called OnlineSubsystem, which is common interface for all online services, including consoles (which support you licance for) and Mobiles + Facebook and Amazon. But i think only Steam on PC supports cloud storage. Some OnlineSubsystem features are supported in Blueprints but i don’t think online storage is one of them you would need to use C++. Here docs:
Big problem with Steam is you can only use it (in distribution) if you get your own AppID for your game from Valve and you can only do that by getting greenlighted… or pay them… a lot 
Alternativly you could use your own solution with HTTP