Introduce PolicyConfigurationAssignedEvent and bulk sync flag

What does this MR do and why?

Introduce PolicyConfigurationAssignedEvent and a feature flag that suppresses individual per-policy events when a security policy configuration is assigned to a project or group.

When a policy project is assigned, SyncScanPoliciesWorker triggers PersistSecurityPoliciesWorker, which currently publishes individual PolicyCreatedEvent / PolicyUpdatedEvent / PolicyDeletedEvent events for every policy. For large configurations this fans out into many small jobs. This MR lays the groundwork for replacing that with a single bulk event that fans out once per project.

Behind the sync_policies_in_bulk_on_policy_configuration_assign flag (disabled by default):

• PersistPolicyService skips EventPublisher#publish (individual policy events) when triggered_by_assign: true
• PersistSecurityPoliciesWorker instead publishes a single PolicyConfigurationAssignedEvent
• SyncPolicyConfigurationWorker is registered as the subscriber (no-op stub; fan-out logic in the follow-up MR)

When the flag is disabled, all existing behavior is unchanged.

References

Issue: #600265

This MR is part of a series:

• 👉 Introduce PolicyConfigurationAssignedEvent and bulk sync flag (this MR)
• Sync policies on policy configuration assign (follow-up MR, targets this one)

How to set up and validate locally

1. Enable the feature flag:

   bin/rails runner "Feature.enable(:sync_policies_in_bulk_on_policy_configuration_assign)"

2. Assign a security policy project to a project via Settings > Security and compliance > Security policies.

3. In Sidekiq logs, verify PolicyCreatedEvent / PolicyUpdatedEvent are not published during the assign flow, and PolicyConfigurationAssignedEvent is published.

4. Disable the flag and repeat — individual policy events should fire as before.

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