Split webhook logic from MergeRequests::RefreshService

What does this MR do and why?

Extract web hooks execution from MergeRequests::RefreshService into separate service and worker

This MR continues the refactoring of MergeRequests::RefreshService by extracting web hooks execution into dedicated components to improve performance and separation of concerns.

Key changes:

New Components:

• MergeRequests::Refresh::WebHooksService - Handles merge request webhook execution for branch updates
• MergeRequests::Refresh::WebHooksWorker - Processes webhook execution asynchronously in the background

Feature Flag Integration:

• Introduces split_refresh_worker_web_hooks feature flag for gradual rollout
• When enabled: webhook execution moves to background worker
• When disabled: maintains current synchronous behavior for backward compatibility

Performance Benefits:

• Reduces blocking time in the main refresh flow by moving webhook execution to background processing
• Improves scalability of merge request operations
• Maintains idempotency and proper error handling

Implementation Details:

The new WebHooksService inherits from BaseService and:

• Creates a Gitlab::Git::Push object with the provided revision information
• Iterates through merge requests for the source branch
• Executes webhook calls with 'update' action and old revision context

The WebHooksWorker is configured with:

• Low urgency and CPU resource boundary (weight: 2)
• Idempotent execution with deduplication until executed
• Database health signal deferral for improved reliability
• Proper error handling for missing projects/users

This refactoring is part of the broader effort to split MergeRequests::RefreshService into focused, maintainable components.

Part of: #554081 (closed)

How to set up and validate locally

1. Create a test project with webhooks:

   • Go to your GitLab instance (usually http://localhost:3000)
   • Create a new project or use an existing one
   • Go to Settings > Webhooks
   • Add a webhook URL (you can use a service like https://webhook.site for testing)
   • Select "Merge request events" trigger
   • Save the webhook

2. Enable the feature flag:

   # In Rails console (gdk rails console) Feature.enable(:split_refresh_worker_web_hooks)

3. Create a merge request:

   • Create a new branch in your test project
   • Make some commits
   • Create a merge request from this branch to main/master

4. Check in the web hook site that the hooks pushed

5. **Check in the sidekiq.log that the new webhook worker is executed