feat: Add ServiceNow integration

[ConeDjordjic]

Implements #3006

Summary

Adds the ServiceNow integration with:

• On Incident trigger — receives incident events via webhook
• Create Incident action — creates incidents via the Table API

Supports Basic Auth and OAuth. Categories, subcategories,
assignment groups, and users are loaded from the ServiceNow API. Selecting a
group filters the user list. Selecting a category filters the subcategory list.

The On Incident trigger requires a one-time Business Rule setup in ServiceNow.
The setup instructions and a ready-to-use script are shown in the trigger
configuration panel.

Note: Automatically creating Business Rules via the ServiceNow API is
possible but requires the admin role on the integration account. The
current implementation uses a manual setup approach to avoid requiring
elevated permissions.

Video Demo

https://www.youtube.com/watch?v=dJuxYJPXyUs

Implementation

Backend (pkg/integrations/servicenow/)

• servicenow.go — integration config and auth
• on_incident.go — webhook trigger with secret verification
• create_incident.go — create incident action
• client.go — ServiceNow Table API client
• list_resources.go — loads categories, subcategories, groups, and users

Frontend (web_src/src/pages/workflowv2/mappers/servicenow/)

• Trigger renderer with Business Rule setup UI
• Action mapper for Create Incident

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