Add background upload

[john-work-ios]

Hi, Is it possible to add support for background upload by setting a URLSessionConfiguration as background?

Would be awesome, thanks!

[skjiisa]

I can look into adding that when I have some time. Feel free to make a PR if you have an idea how to implement it.

[john-work-ios]

Thank you for the reply. I'll see what I can do and create a PR.