* Create ProxyConfig class + Unit tests
* Use ProxyConfig in SpotifyLocalAPI
* Refactor WebProxy creation in ProxyConfig
* Use ProxyConfig in SpotifyWebAPI
* Add ProxyConfig parameter in GetAlbumArt methods
* Add ProxyConfig to WebAPIFactory
* Add proxy settings to Example app
* Fix ArgumentNullException in GetAlbumArtUrl when not using proxy
* Performed requested changes
- Removed redundant "this".
- Added necessary null checks.
- Added a ProxyConfig property to SpotifyLocalAPIConfig and removed constructor overloads with it in SpotifyLocalAPI and RemoteHandler.
* Updated Example app
