Add ability to cancel uploads and downloads

[sujaygarlanka]

Goals ⚽

• Add ability to cancel uploads and downloads
• Resolves How to cancel a file download session #709

Implementation Details 🚧

• A BoxDownloadTask returned for every download request. This task contains the URLSessionTask that is associated with that request. BoxDownloadTask has a cancel method on it that can be called. When called, it cancels the URLSessionTask associated with, resulting in a cancelled request.
• A BoxUploadTask returned for every upload request. BoxUploadTask has a cancel method on it that can be called.

Testing Details 🔍

• Added unit tests

[jlawton]

This looks like it works to me, and seems like a reasonable evolution of the code.

I think that Combine (which I know we can't use) has a slightly cleaner solution to this sort of thing, but it hinges on the fact that Publishers separate initialization and subscription, and that they are chained, rather than using callbacks. I think moving in that direction would be too large of a change.

This wouldn't be totally trivial to implement. Just to document the idea in case you're interested in an alternative, it would look something like:

class NetworkTask<T> {
    /// Run a task
    func subscribe(_ completion: @escaping Callback<T>) -> Cancellable
    /// Chain tasks
    func then<U>(_ nextTask: (T) -> NetworkTask<U>) -> NetworkTask<U>
}

func preflightCheck(…) -> NetworkTask<Void>
func upload(…) -> NetworkTask<File>

func uploadWithPreflightCheck(…) -> NetworkTask<File> {
    return preflightCheck(…).then { upload(…) }
}

func uploadWithPreflightCheck(…, _ completion: @escaping Callback<File>) -> Cancellable {
    return uploadWithPreflightCheck(…).subscribe(completion)
}