[ResponseOps][Connectors] Highlight step as optional in connector cre…

…ation form (elastic#191918)

## Summary

I have added the `(optional)` text to step 4 of the `Webhook - Case
management` creation form.


### Before

![before](https://github.com/user-attachments/assets/f0e7e76b-335f-4ac7-bcb0-a179ec222c0b)

### After

<img width="1726" alt="Screenshot 2024-09-02 at 14 32 39"
src="https://github.com/user-attachments/assets/1498f5e9-1827-4cd1-a91f-5748caff0dbe">

Co-authored-by: Elastic Machine <elasticmachine@users.noreply.github.com>

x-pack/plugins/stack_connectors/public/connector_types/cases_webhook/translations.ts

@@ -340,7 +340,7 @@ export const STEP_4A_DESCRIPTION = i18n.translate(
 );
 
 export const STEP_4B = i18n.translate('xpack.stackConnectors.components.casesWebhook.step4b', {
-  defaultMessage: 'Add comment in case',
+  defaultMessage: 'Add comment in case (optional)',
 });
 
 export const STEP_4B_DESCRIPTION = i18n.translate(