add URL for HockeyApps identification site to public interface

Classes/BITAuthenticator.h

@@ -220,6 +220,12 @@ typedef NS_ENUM(NSUInteger, BITAuthenticatorAppRestrictionEnforcementFrequency)
  */
 @property (nonatomic, strong) NSURL *webpageURL;
 
+/**
+ * URL to query the device's id via external webpage
+ * Built with the baseURL set in `webpageURL`.
+ */
+- (NSURL*) deviceAuthenticationURL;
+
 /**
  * The url-scheme used to idenfify via `BITAuthenticatorIdentificationTypeDevice`
  *