macOS support?

* [X] I've read, understood, and done my best to follow the [*CONTRIBUTING guidelines](https://github.com/box/box-swift-sdk/blob/master/CONTRIBUTING.md).

## What did you do?

I attempted to integrate a macOS app with the Box SDK.

## What did you expect to happen?

I realize that the name of the SDK is `box-ios-sdk`, but `osx` is listed as one of the supported platforms. As such, I expected the SDK to work with a Mac app.


## What happened instead?

The project links against the SDK without error, but then things start failing silently. I attempted using sample code to authenticate via OAuth and the call never returns. In looking at `obtainAuthorizationCodeFromWebSession`, I discovered that the code simply bails if not iOS. This is a silent fail without any console log if running on another platform.

```
func obtainAuthorizationCodeFromWebSession(completion: @escaping Callback) {
let authorizeURL = makeAuthorizeURL(state: nonce)
#if os(iOS)
... iOS goodness here...
#endif
}
```

My preference would be for the SDK to work with Mac (like [Dropbox](https://github.com/dropbox/SwiftyDropbox) does). If Box chooses to not go that direction, the code should be updated to log a console message and/or return an error in the callback and documentation should be updated so others don't spend time going down this same path.

## BoxSDK Environment

**BoxSDK version:**
3.1.1

**Xcode version:**
11.3.1

**Swift version:**
5

**Platform(s) running BoxSDK:**
macOS

**macOS version running Xcode:**
10.15.2

## Demo Project
[clouduploader.zip](https://github.com/box/box-ios-sdk/files/4140356/clouduploader.zip)
Client secrets and IDs scrubbed from the above for both Box and Dropbox.

Start by reviewing obtainAuthorizationCodeFromWebSession and its #if os(iOS) branch, then reproduce the silent OAuth failure using the linked demo project on macOS. The issue needs a project decision first: support macOS authentication, or define and document an explicit error path for unsupported platforms.

Written by the indexing model from the issue text.