login method
login opens Snapchat's OAuth screen in-app or through a browser if
Snapchat is not installed. It will then return the logged in SnapchatUser
An error will be thrown if something goes wrong
Implementation
Future<SnapchatUser> login() async {
await _channel.invokeMethod('callLogin');
final currentUser = await this.currentUser;
this._authStatusController.add(currentUser);
this._authStateListener?.onLogin(currentUser);
return currentUser;
}