You will need a separate amp, or a receiver which can assign a spare channel to the subwoofer output. This isn't a common feature as most often subwoofers come with a powered amp already. If this isn't the case, you will need to just pick up an amp for that subwoofer. Then you can pair it with any receiver you would like.
Normally I recommend getting separate amps for outdoor speakers, but if this area is to be it's own space, run independently from the rest of the home, then I would pick up a modestly priced current generation AV receiver that supports 'all channel' stereo. Denon and Yamaha typically do, so this shouldn't be an issue. Both Denon and Yamaha support WiFi control of their products and most have integrated Bluetooth, but the range of Bluetooth will make it pretty useless from an inside to outside usage scenario. WiFi is the way you will want to go. Make sure to have an access point outside which can service your entire backyard area properly.
From there, I think you would be pretty setup, and you can add sources if you would like any additional functionality.
I would add that if you ever have a location where you may even consider adding a TV outside in the future, you should go ahead and wire for that right now. Pull a couple of CAT6 cables and put a power outlet nearby. You may never use it, but it's a pretty cheap piece to put in during construction and can tie into the receiver easily enough.
