Audit Events: Log feature flag enable/disable if actor is present

Problem to solve

Feature flag changes can affect the security of users and organizations. However, they are not logged as audit events. This was reported by a Premium customer and also noted by GitLab internally.

Proposal

Feature flag enable and disable should be logged as individual Audit Events.

Proposed Solution

1. Make Gitlab::Audit::RailsConsoleAuthor class, Use this for author data in audit_event.
2. For Target use Feature::FlipperGate.
3. Store actor as scope for audit event.
4. Use Auditor Framework to generate audit events. refer: https://docs.gitlab.com/ee/development/audit_event_guide/#how-to-instrument-new-audit-events