feat: accept and expose virtual_key_url param in auth response

[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 when redirecting back with the authorization code. This PR updates the SmartcarResponse class to include a new instance variable, virtualKeyUrl, with the corresponding getter/builder methods to access and set the virtualKeyUrl value.

From there, the virtualKeyUrl can be used within the SmartcarCallback 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

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (805f481) 91.27% compared to head (a67fe71) 91.61%. Report is 1 commits behind head on master.

:exclamation: Current head a67fe71 differs from pull request most recent head e690873. Consider uploading reports for the commit e690873 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #31 +/- ## ========================================== + Coverage 91.27% 91.61% +0.33% ========================================== Files 4 4 Lines 149 155 +6 Branches 9 9 ========================================== + Hits 136 142 +6 Misses 11 11 Partials 2 2 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.