Audit duo namespace feature access rule updates

What does this MR do and why?

Create audit events for updates to feature access rules at the top-level namespace

Since much of this change mirrors the auditing added in !217749 (merged) for Self-managed, the changes in this MR also include refactoring the auditing logic into Ai::FeatureAccessRuleAuditor.

Depends on !216946 (merged)

References

• https://gitlab.com/gitlab-org/gitlab/-/work_items/584381+

Screenshots or screen recordings

How to set up and validate locally

1. Run GDK in SaaS mode: export GITLAB_SIMULATE_SAAS=1

2. In the rails console, enable the feature flag for a top-level namespace: Feature.enable(:duo_access_through_namespaces, root_namespace)

   1. Create feature namespace access rules

      curl -w 'http://gdk.test:3000/api/v4/groups/1000000' \
        -X 'PUT' \
        -H "Authorization: Bearer $GITLAB_TOKEN" \
        -H 'Content-Type: application/json' \
        --data-raw '{
          "duo_namespace_access_rules": [
            { "through_namespace": { "id": 963 }, "features": ["duo_classic", "duo_agent_platform"] },
            { "through_namespace": { "id": 964 }, "features": ["duo_agent_platform"] }
          ]
        }'

3. Check Audit events to verify that the event was created with correct information

   AuditEvent.last

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Related to #584381