ExposureNotification iOS 14.3 to 14.4 API Differences

Added #def EN_API_AVAILABLE_V3

Added ENActivityFlags

Added ENActivityFlagsReserved1

Added ENActivityFlagsReserved2

Added ENActivityFlagsPeriodicRun

Added ENActivityFlagsPreAuthorizedKeyReleaseNotificationTapped

Added ENActivityHandler

Added ENDiagnosisKeysAvailableHandler

Added ENManager.activityHandler

Added ENManager.diagnosisKeysAvailableHandler

Added -[ENManager preAuthorizeDiagnosisKeysWithCompletionHandler:]

Added -[ENManager requestPreAuthorizedDiagnosisKeysWithCompletionHandler:]