BREAKING CHANGE: add virtualKeyUrl to completion handler arguments

[allisonc07]

For brands that require a virtual key to be registered on the vehicle in order to receive data and/or send commands to the vehicle, Smartcar will be sending back an additional query parameter, virutal_key_url, when redirecting back with the authorization code. This PR updates the interface of the completionHandler to include a new parameter, virtualKeyUrl, and to pass the value of the virtual_key_url flag to the completionHandler when present.

From there, the virtualKeyUrl can be used within the completionHandler that is created by the developer, and passed onto the BE, etc. so that it can be sent to the appropriate vehicle owners.

[codecov[bot]]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (6964c98) 0.00% compared to head (1c4e3d8) 0.00%.

Files	Patch %	Lines
SmartcarAuth/SmartcarAuth.swift	0.00%	6 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #76 +/- ## ====================================== Coverage 0.00% 0.00% ====================================== Files 4 4 Lines 140 141 +1 ====================================== - Misses 140 141 +1 ```

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