Add: missing notification Level urgent (#295)

## What
- Add missing notification urgent Level
- Add AllowedLevels slice

## Why
To cover all possible notification Levels

## References
- greenbone/opensight-notification-service#200
- https://jira.greenbone.net/browse/VTI-784

## Checklist

Author: alex-heine

pkg/httpassert/request.go

@@ -132,6 +132,7 @@ func (s *starter) Getf(format string, a ...interface{}) Request {
 func (s *starter) Options(path string) Request {
 	return s.newRequest(http.MethodOptions, path)
 }
+
 func (s *starter) Optionsf(format string, a ...interface{}) Request {
 	return s.newRequest(http.MethodOptions, fmt.Sprintf(format, a...))
 }

pkg/notifications/model.go

@@ -39,8 +39,16 @@ const (
 	LevelInfo    Level = "info"
 	LevelWarning Level = "warning"
 	LevelError   Level = "error"
+	LevelUrgent  Level = "urgent"
 )
 
+var AllowedLevels = []Level{
+	LevelInfo,
+	LevelWarning,
+	LevelError,
+	LevelUrgent,
+}
+
 // toNotificationModel converts a Notification to the rest model for the notification service.
 // It also adds the current time as timestamp if it is not set.
 func toNotificationModel(n Notification) notificationModel {